Lag time and lead time help you create more complex relationships between linked tasks.
Life can be complicated. What if your plan needs to include complex task relationships? In Project, you can build in a delay between the finish of one task and the start of another. Or you can make one task overlap with another.
For delay, use lag time. This accounts for something that must happen between two tasks, such as waiting for a shipment of supplies to arrive.
For overlap, use lead time. If a task can start a week before the previous one is finished, a lead time of seven days gets that second ball rolling.
As you'd expect, lead time and lag time adjust to match other changes. The start date may change, the finish date may change, but the next task stays linked and scheduled right where you want it.