what skipped Assignments are and how they appear ?

Hello,

Context: a Learning specialist has identified and purged “skipped assignments”. They no longer appear in the plan as errors.
However, this Learning specialist doesn't know why the entry of these assignments generated “skipped assignments” which gave rise to this “error” status (she had never explored “Search for skipped assignments”).
Can someone help me understand what skipped Assignments are and how they appear, and in which cases this “Search for skipped Assignments” feature should be used?
And if the solution is indeed to purge them? Is it possible to have automatic or automated purging?

Thank you in advance for your help.