To "reposition" is to move from one place to another, but not as a revenue activity. For aircraft, this is sometimes referred to as "ferry". For crews, this is either "deadheading" (flying in a passenger seat) or "jumpseating" (flying in the cockpit jumpseat).
Flight crews live all over the world. Some have to "commute" to get to the airports where they're based, and crews (especially ones on reserve) often get assigned to operate trips that don't start/end at their base anyway. Airlines naturally prefer to move crews on their own flights, and will even bump paying passengers to do so if needed, but there is a web of agreements letting them use each others' flights as well if space permits.
Cargo airlines have pretty limited schedules, often operating only overnight, so to get their crews to/from their planes in a more reasonable timeframe, they may reposition on passenger airlines. OTOH, a passenger crew might use a freight flight to reposition overnight to avoid an overnight in a hotel before the first passenger flight of the day or after the last.