If you are hanging out with a couple of friends, you are hanging out with two friends. A couple is a pair or a twosome. As a verb, couple means "to pair or match."
As a noun, couple means "a pair." If two people are dating, you can refer to them as "a couple." Similarly, two people who are married can be called a "married couple." Two guys playing basketball can be called "a couple of guys playing basketball." As a verb, couple means "to pair or combine," as people on a dance floor might couple or form pairs when the music starts. You can also couple two ideas by combining them.