Background is what’s behind something — whether it’s physical, like a play’s scenery, or intangible, like a person's life story or the past facts of a current situation. If you are shy, you might try to stay in the background at parties and avoid attention.
The original idea of background was the use of stage scenery or painted drops that established a play's setting. Later, the idea expanded to include the figurative meaning of something that was present yet not immediately noticed, such as the background music in a movie. It also came to mean material that happened earlier and contributed to something. If your new boss does a "background check" on you, it's because she wants to know all the trouble you've caused in the past.