Leveling is a part of the mixing process.
It’s when you first start out in your mix, you try to adjust the volumes of each instrument before apply effects; you’re just “leveling” out the volume’s of each instrument to get a balanced sound.
Then in the mixing process, you can start applying your effects to get the sound you want.
Mixing is when we adjust each individual instrument until we are happy with how it sounds inside of the whole song.
Mastering is when we finally work on the mix on the 2-bus to polish up this mix for commercial release!