Free Slack: The amount of time that can be spared in a task before it begins to affect other tasks.
Some tasks don’t really need to be completed by the time you’ve set for them. In other words, there’s a little slack available before they need to be finished. That’s Free Slack.
Microsoft Project calculates free slack in tasks when they are linked to other tasks. If a task is not linked to another, the free slack is the amount of time from the finish date until the end of the project. Here’s a quote from MSP:
The Free Slack field contains the amount of time that a task can be delayed without delaying any successor (successor: A task that cannot start or finish until another task starts or finishes.) tasks. If the task has no successors, free slack is the amount of time that a task can be delayed without delaying the entire project’s finish date.
So, how is this valuable to you? This only applies when a successor task is not linked directly to its predicessor. In other words, there is some slack time between them, even though they are technically linked. This can be valuable to offer some spare time for the resource to finish the task, or to do other things.