Monitoring is listening to your sound on speakers or headphones while recording, editing and mixing. Because it is the basis for your decisions, level and environment matter.
The basic idea
Listening loud for a long time makes your ears adapt, and judgments about the low and high ranges become unreliable.
What to check when doing it
Check in several ways: at a lower level, on headphones, and on a different speaker.
It refers to listening in order to make production decisions, not to playback in general.
