Explanation:

The effects of the Z gate can be seen when applied to the |+〉 state in the following visualization, where a rotation of π about the z axis occurs. After the transformation, the final state is |-〉, and when measured, still has an equal probability of being 0 or 1. The Z gate performs a rotation of π radians around the z axis.

(think of it like just going backwards on a line graph on the x axis where numbers become negative)