What does Rebuild All Task do?

Q: What will happen when I choose the red box to Rebuild all Tasks?

A: Checking the red box along with checking the box(es) to Update Tasks will redo the schedule for that specific Task Rule for any property(ies) that are selected to Update Tasks.  

This is mostly used when setting up a new account and an existing schedule has not been put into place.  In most cases, if changes are being made in Setup --> Task Rules; Edit Assignments the yellow box is used. 

Here is more information Update Assignments that have Tasks Generated