Definitions of background:

• noun:   (computer science) the area of the screen in graphical user interfaces against which icons and windows appear
• noun:   a person's social heritage: previous experience or training

  Example: "He is a lawyer with a sports background"

• noun:   information that is essential to understanding a situation or problem

  Example: "The embassy filled him in on the background of the incident"

• noun:   the part of a scene (or picture) that lies behind objects in the foreground

  Example: "He posed her against a background of rolling hills"

• noun:   relatively unimportant or inconspicuous accompanying situation

  Example: "When the rain came he could hear the sound of thunder in the background"

• noun:   extraneous signals that can be confused with the phenomenon to be observed or measured

  Example: "They got a bad connection and could hardly hear one another over the background signals"

• noun:   scenery hung at back of stage
• noun:   the state of the environment in which a situation exists
• verb:   understate the importance or quality of