Chicken can mean a few different things:

1. A common farm bird raised for its eggs and meat. Chickens are domesticated descendants of wild jungle fowl from Southeast Asia, and humans have kept them for thousands of years. A female chicken is called a hen, while a male is a rooster. Baby chickens are chicks. Chickens scratch in the dirt looking for seeds and insects, roost in trees or coops at night, and make distinctive clucking and crowing sounds. They're found on farms and in backyards almost everywhere in the world.
2. Someone who's afraid to do something, or the act of being afraid. If your friend dares you to jump off the high dive but you back out, they might call you chicken. You might chicken out of giving a speech in front of the class if your nerves get the best of you. This meaning probably comes from the bird's reputation for being nervous and easily frightened, though real chickens are actually braver than most people think.

The phrase don't count your chickens before they hatch means don't assume something good will happen until it actually does, like celebrating an A on your science project before your teacher has even graded it.