Sound Mixers
A device that mixes two audio signals together is called a "mixer." Mixers have two or more faders that will allow you to adjust the incoming volume level of each audio source.
There's only one really important rule here--but it's critical: At any given point in time, only ONE audio source can be dominant. It can be voice, music, sound effects, whatever--as long as only ONE is loud (full volume). All others must be quiet (in the background). In a given piece, the dominant audio source might change several times. For example, the narration might stop and then the background music would get louder and become dominant. But at any given point, only ONE is full volume.
Simple Mix
To the left is an example of how a mixer might be used: In the clip there are two audio sources: music and voice-over. If they are mixed at the same level, it quickly becomes clear that the music is too loud and drowns out the voice-over. So the mixer is used to reduce the volume of the music until it no longer drowns out the voice. (Play the clip to hear how this happens).
Some edit software now calls this process "ducking" but that's not a common term among professionals.
A professional mixing board might have 16 or 24 faders to control lots of incoming audio, but the process is the same as in this simple example.