The indirect effect means that the variable (X) passes through the mediator (M) to cause its effect on the outcome (Y). Said another way, the variable (X) affects the outcome (Y) by way of the mediator (M). The indirect effect can be expressed as an amount of mediation (a measured magnitude) by the mediator (M).

In the context of the causal diagram, the magnitude of the mediation can be observed when the variable (X) will result in the increase on the outcome (Y) while holding the variable (X) constant, while at the same time, when a unit increase in the variable (X) would achieve an increase in the mediator (M).