What is a simple system of local coefficients?

Question

In the context of spectral sequences of fibrations I have often seen the condition that a fibration has a simple system of local coefficients.

Unfortunately I don't know what this means, and google searches are not helpful.

What does it mean for a fibration to have a simple system of local coefficients?

Answer 1

One on which the $\pi_1$ of the base acts trivially.