How to reset timer if state changes?

[JKnight (Limping this along since 2013!)]

@rlcronin

what preconditions are

Here is an good explanation of what preconditions are:

What is precondition?

Im new to creating routines in smartthings. Can someone explain what the button “Use as precondition” means simply? It appears when I add something in the if part of the routine such as a thermostat temp. I dont see how precondition differs from it being not selected, but the item is still in the if section.