Overlapping shifts allows volunteers to sign up for shifts that may be identical or have a second (partial) shift overlapping.
This is an option you can turn on or off under Events & Programs > Events & Programs > Edit Detail.
Scroll to Shift Conflicts, there will be a dropdown to select the type of overlapping allowed for volunteers.
There are four options:
Do not allow overlapping shifts, and enforce a gap of: this will enable you to put buffers between shifts so the volunteer cannot have a shift within that gap time
Allow overlapping shifts, but limit the overlap to: this setting enables volunteers to overlap shifts but only the allowed amount of time set