What does the emergency exit sign mean?

An "emergency exit" is a door or passageway to a place that should be safer than where you are. It is "emergency" exit because it would not normally be open for use (going in or out), but must be able to open during an emergency.

All emergency exits are required to be marked with an illuminated "EXIT" sign. Your local fire code will determine how many there must be, whether they are required to have "panic hardware" to unlock them (i.e., by a blinded child with no hands), among other things.

In countries other than the USA, you may see "Way Out" signs, rather than EXIT, or a pictogram showing a person going through a door or up/down stairs.