Slack Time is the length of time between the project task's planned end date and the task's deadline date. Showing Slack Time is a good way of seeing, graphically, task deadlines and the amount of slippage that can safely occur.
The picture below shows the Project window displaying Slack Time and containing several tasks, each of which have been assigned deadlines.
To display slack time, select Show Slack from the View menu. To hide slack time, select Hide Slack from the View menu.