The concept can be described using the following analogy: when Romeo loves Julia AND Julia loves Romeo, there is reciprocity or mutualness in that love. What Romeo gets from Julia, Julia gets from Romeo. What Romeo gives to Julia, Julia gives to Romeo.
In a simple diagram you could have two symbols for Romeo and Julia and two arrows between them for the love stream. The fact that there are two identical arrows to and fro two entities completes the reciprocity of that relation.
If Romeo's website links to Julia's website AND Julia's website links to Romeo's website, the websites are reciprocally linked.