|Ergonomics is, basically, the science of comfort. It is the study of how your equipment affects you when you are at work, both in terms of maximising your efficiency and reducing any health risks that might be present.
Where you've probably heard the term, though, is used in relation to computer equipment: mice, mouse mats, keyboards, monitors, and the like. The traditional design of a lot of computer equipment is very unergonomic, meaning that if it is used for extended periods of time (as you do when you are using it at your job), you can get all sorts of injuries, the most famous being RSI (repetitive strain injury).
Whatever you might think of your employer, the chances are that they don't want you getting injured: they'd have to get along without you, or go to the trouble of replacing you, not to mention the legal liability they might have if you could prove that it was there equipment that hurt you. For this reason, ergonomic computer equipment is becoming more and more common in office environments. If your keyboard is oddly-shaped, that's because it's ergonomic. If your mouse mat has a little soft spot to rest your wrist on, that's because it's ergonomic. There are plenty more examples.
It's these little, inexpensive extras that go some way towards reducing the possibility that you will get injured at work. One thing you might not have considered, though, is that they can also work well in the home. If you use your computer a lot at home - whether as a hobby, or as a freelancer, or for home working - you should get some ergonomic equipment for yourself. If you injure yourself through computer use, you'll have no one to sue or blame but yourself, which might mean that you won't be able to afford the costs of caring for and treating the condition. RSI really isn't pleasant, so you should make sure to take measures to protect yourself, wherever the computer you're using might be.