A lot of what people will call "empathy" is really "sympathy" -- a philosophical fellow-feeling for someone else's emotional state.
Empathy is when you feel what someone else feels, in one of two ways. Think of it like perceiving the heat from a fire. If you hold out your hand, you will feel that there is heat coming from the fire. If you keep your hand there for a while, then your skin will start to heat up and you will become warmer yourself. So you can sense emotions coming from someone else, which can feel like heat or a fizz of energy or many other things; and those emotions can soak into you so that you begin having the same emotion as those around you.
This can work in various ways. One is that living things give off energy, and each person generates an electromagnetic field. That energy can convey emotions. Empaths are more sensitive to the energies coming from other people.
Mirror neurons are brain cells that react the same way to the experiences of others as they do to our own experiences. This is what enables things like sympathy, compassion, and conscience because of that sense that other people are real and can feel pain. Without it, you get psychopaths who feel like they are the only real person and everyone else is just a puppet. An empath might have more mirror neurons, or more effective ones, and therefore be much better at detecting the emotions of others.
There seem to be different types of empathy. Some people can only read others in person at relatively close range. Some have a much longer range. Some can read anyone they know personally, including over the phone or computer, or even without contact at all. Some can read through recordings such as television, others not. A few are even better at it long-distance than in person.
Here are some tips for interacting with empaths.
1) Basic civility helps a lot. If someone asks you to quit doing something, then quit it unless you have a compelling reason for doing it. If you hurt someone's feelings, apologize. Don't slop your personal issues over nonconsenting bystanders.
2) Proximity and especially skin contact enhance many abilities, including empathy. For this reason, many empaths are cautious about touching other people, so don't grab them or lean on them. However, some empaths are the opposite, extremely tactile, because they need the contact for reassurance or comfort. Try to accommodate their needs too.
3) Shielding is what keeps people's energy from merging. Ideally, boundaries should be semi-permeable, not vaporous or rigid. Empaths who can't learn to shield tend to become hermits if they are free to do so, or go mad if not free, because the pressure of emotions is overwhelming. Even empaths who can shield are prone to getting all peopled out faster because it takes energy to moderate that input. Conversely, you can help by keeping your feelings to yourself instead of hosing the room with them. It's not about hiding your emotions, but rather modulating them -- like speaking in a normal tone of voice instead of shouting. There are many exercises for shielding.
4) When empaths get upset, some of them need to be alone, but others need to be around healthy emotions. So you might be able to cheer one up by thinking happy thoughts or watching a funny movie together.
5) Empaths can zone on emotions, which means getting fixated on a single emotional source. When others are in a very intense state of stimulation, empaths may overflow in response -- or they may seem to shut down and go blank. You might be able to jostle them out of it by bumping them gently and projecting a different emotion. Sometimes they may need to leave the room for a while, in which case, you can help by walking them out. If they're really overwhelmed they sometimes have trouble navigating or staying safe. Many of the tips for helping someone through a sensory meltdown also help with empathic overload or zoning.
6) Watch to see how reliable people's instincts are. Empaths often have very accurate hunches long before anyone else can pin down concrete reasons why John is a creeper or Jane has sticky fingers. So if you know an empath who clocks people very well, back them up. Make sure nobody's alone with John and things are put away before Jane arrives. It's exhausting to keep giving warnings only to get grilled for proof you don't have, and maddening to let bad things happen because nobody will listen. Friends who offer support are precious. Validation is a fundamental human need.
People have varying amounts of awareness and sharing of other's emotions, and thus, different levels of responsiveness and interpersonal skills. Emotional intelligence is important and there are exercises for improving it.
I suspect that some folks have less mirror neurons than average and/or theirs don't work as reliably. "Headblind" is a term for someone with little or no psychic sense, and it also applies to people who just lack that awareness of others' feelings that humans expect. There are different things going on here:
* Ability to sense input.
* Ability to interpret those perceptions.
* Knowledge of other people -- these individuals, and how humans in general behave.
Someone may have low or erratic ability to perceive or to interpret emotional information; or they may have difficulty understanding people. They may sense something quite clearly, but if they don't know what it means, then it may not be very useful for predictions. Or they may not pick up the signals at all.
If someone doesn't have this awareness, there's no more point yelling at them for being insensitive than in yelling at a blind person for crossing a painted line they can't see. It's better when people compensate for each other's weaknesses.