Lag time is a delay between dependent tasks.
For example, suppose you plan to start editing a manual three days after writing starts. For the "Edit Manual" task, you'd enter 3 in the Lag field on the Predecessors tab in the Task Information dialog box, and then click OK. You can open the dialog box by double-clicking the successor task.
Note Days are the default unit of time for lag time, or they can be entered specifically as D or d. You can enter different units of time by typing them: M or m for minutes, H or h for hours, W or w for weeks, and Mo or mo for months.
You can also enter lag time directly in the Predecessors column of the successor task in the chart portion of the Gantt Chart view. If "Write Manual" is Task 1 and "Edit Manual" is Task 2, in the Predecessors column of Task 2, you could type 1SS3.