What does the Master button do?

The Master button applies a mastering effect chain to the selected audio. It changes volume, dynamics, and loudness while targeting a streaming-friendly loudness level measured in LUFS. You can try the workflow on the Automatic Mastering page.

Mastering is usually the final step after editing, restoring, EQing, or mixing. It does not magically fix every stem separately. If you are working with stems, finish your stem edits first, export the mixed result, then import that mix again and master it as one full track.

For the detailed version, see whether you should master each stem separately.