Reciprocal pronouns are pronouns used when referring to two or more people or things that both participated in the same action and both received the benefit or effect of that same verb. Since they are receiving the action or effect of the verb, reciprocal pronouns are always objects. This definition probably sounds confusing, but it will make much more sense once you see them in action.
Reciprocal pronouns are easy to remember because there are only two: each other and one another. Traditionally each other was used when referring to two people or things, and one another was used when referring to three or more people or things. Some people still like to follow the old rule, but one another and each other are now generally used interchangeably.
We'll all call one other this weekend. (direct object) The whole class gave each other gifts at the party. (indirect object) Be kind to one another. (object of the preposition)