Lead time is overlap between dependent tasks.
For example, if the "Prepare Binders" task can start two days before its predecessor task "Print Manual" is finished, you can create a lead time of two days, and work on the tasks will overlap, proceeding at the same time.
Lead time is entered as a negative value in the Lag field on the Predecessors tab in the Task Information dialog box. You can open the dialog box by double-clicking the successor task in the tasks list.
For this example, you'd double-click "Prepare Binders" to open the dialog box, select the Predecessors tab, then type -2 in the Lag field. Finally, you'd click OK.
You can also enter lead time directly into the Predecessors column of the successor task in the chart portion of the Gantt Chart view. If "Print Manual" is Task 4, and "Prepare Binders" is Task 5, in the Predecessors column of Task 5, you'd type 4FF-2.