A mediator is a variable that is influenced by the x-variable and influences the y-variable. In other words, some (it could be a little or a lot) of the effect of x on y is mediated through z. 

Example We want to examine the association between occupational class (x) and liver cirrhosis (y). We think that the association may be mediated by alcohol consumption (z).  We are interested in the association between poverty (x) and all-cause mortality (y). It is reasonable that this association might to some extent be mediated by stress (z).