Source music refers to music that is part of a fictional setting, such as film

Film

A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...

s or video games. It is usually either played in this setting in the background (e.g. in a radio or TV) or by characters of the medium (e.g. as part of the plot). The opposite of source music is underscoring

Underscoring

In film production, underscoring is the playing of music quietly under dialogue or a visual scene. It is usually done to establish a mood or theme. In a play, sometimes incidental music is used for this purpose....

which is played only for the viewer or player of the medium and is not part of the fictional setting itself.