Music with wide dynamic range moves from whisper-quiet to very loud, like orchestral film scores. Narrow dynamic range keeps everything at a similar level, typical of pop and electronic music mastered for loud playback.
A common measure is loudness range (LRA), shown in LU. Low values suit background beds; high values suit dramatic standalone listening.
Using it in a prompt
Ask for "even dynamics" for beds under voice and "big dynamic build" for trailers and emotional peaks.