To get rid of something is to throw it away or otherwise dispose of it. You may need to get rid of the garbage stinking up your kitchen, or you may need to get rid of your annoying neighbor during your backyard BBQ.
When you get rid of clothes that don't fit you, you might give them to a friend or donate them to a charity. If you have to get rid of your guinea pig, you'll want to find her a new home or take her to the local shelter. Sometimes get rid of is used to mean "kill" or "abolish," the way a gangster gets rid of an enemy or a senator gets rid of an archaic law.