It represents a situation where extra resources are available and the completion of project is not delayed

When SLACK of an ACTIVITY is POSITIVE - it represents a situation where extra resources are available and the completion of the project is not delayed.
When slack of an activity is negative- it represents that a program falls behind schedule and additional resources are REQUIRED to complete the project in time.
When slack of an activity is zero - the activity is CRITICAL and any delay in its performance will delay the completion of the whole project.