[common] To do a task in background is to do it whenever foreground matters are not claiming your undivided attention, and to background something means to relegate it to a lower priority. “For now, we'll just print a list of nodes and links; I'm working on the graph-printing problem in background.” Note that this implies ongoing activity but at a reduced level or in spare time, in contrast to mainstream ‘back burner’ (which connotes benign neglect until some future resumption of activity). Some people prefer to use the term for processing that they have queued up for their unconscious minds (a tack that one can often fruitfully take upon encountering an obstacle in creative work). Compare amp off, slopsucker.
Technically, a task running in background is detached from the terminal where it was started (and often running at a lower priority); oppose foreground. Nowadays this term is primarily associated with Unix, but it appears to have been first used in this sense on OS/360.