The mediator is a variable that functions as the mechanism by way of transferring an effect of one thing to another.

Consider the chain junction, where $A -> B -> C$. $B$ is the mediator that transfers the effect of $A$ to $C$. In other words, the mediator, $B$, is the variable that explains the causal effect of $A$ on $C$. $A$ can be an indirect cause of $C$, meaning that $A$ affects $B$, which in turn contributes to the change in $C$.