Schedule Lag is a spare time prescheduled between tasks in order to mitigate a possibility of a task delay: you reserve some spare time for a task to be prepared for a delay (you create a buffer), so if this task gets protracted, then you will have additional time until the project deadline will be affected. This makes sense if a task is associated with high risks, uncertainty, and therefore has a high probability of procrastination. This delay could also happen because of some factors which are out of project management control yet (they may make duration of this task variable). For example a schedule lag can be made to anticipate a time that will be taken for completion of certain legislative procedures as far as their duration can vary due to dependence on workflow of certain governmental institution (let’s say they need a time to consider your request). A schedule which has a big number of lags can be called a “loose schedule” which may get compressed during the project execution as tasks with lags after them can be done without involving this spare time, so the lags might be eliminated (the next tasks will be started earlier).