If you refer to a couple of people or things, you mean two or approximately two of them, although the exact number is not important or you are not sure of it.

A couple is two people who are married, living together, or having a sexual relationship.

A couple is two people that you see together on a particular occasion or that have some association.

If you say that one thing produces a particular effect when it is coupled with another, you mean that the two things combine to produce that effect.

If one piece of equipment is coupled to another, it is joined to it so that the two pieces of equipment work together.