A good schedule will optimize the relationships among activities. Sometimes there must be a time between two activities when no project work is done. In this case, a lag is introduced into the relationship.
An example would be the time between pouring concrete and sanding it. The concrete must have time to dry before the sanding can begin, and this drying time can be represented as the lag between those two activities. There is no work nor resources involved, just time passing.
In another example, excavation can begin five days after a permit is submitted. There would be a 5 day lag on Begin Excavation as a successor to the Submit Permit activity.
In the screen above, activities A and B both have two-day durations, and the start of ...