The amount of time that a non-critical path activity can be delayed without delaying the project is referred to as slack time.
Let’s take a example to understand it better:
We have three tasks T1, T2, T3.
T1 takes 5 units of time and independent of any other task.
T2 takes 7 units of time and independent of any other task.
T3 takes 2 units of time and depends on both T1 and T2.
So the Slack Time for all tasks will be
T1: 2 units of time (T1 can get start 2 units of time late and still the process will complete in the specified time).
T2: 0. (It’s on the critical path!)