Slack time

The amount of time that a non-critical path activity can be delayed without delaying the project is referred to as slack time.

For example,

We have tasks A, B, C

A - 4 days, no dependent
B - 10 days, no dependent
C - 8 days, depends on A and B

Then task A will have slack time of 6 days. B has 0 days. And C has 0 days.