In essence a "restart" is an attempt to restart the HPTA (hypothalamic-pituitary-testicular axis) for a patient who's axis is suppressed or shutdown due to being on TRT or AAS.
It is similar to a "stimulation" type regimen (as opposed to frank TRT), except for the starting point for a restart is typically a FULLY suppressed HPTA due to TRT/AAS, whereas the starting point for a general stimulation regimen is simply a sub-optimal or partially suppressed HPTA for various other factors (not current TRT/AAS use). Semantics really, the first paragraph gives the jist.