The technical distinction between these two critical stages of music production.
A common mistake for independent artists is confusing mixing with mastering. While they are both part of the 'post-production' process, they require different tools, different rooms, and usually different engineers.
Mixing is the art of balance. The engineer takes all the individual tracks—drums, bass, vocals, synths—and blends them together. They use EQ, compression, and effects to ensure every element has its own space in the frequency spectrum and the stereo field.
Mastering is the 'final polish.' It happens after the mix is finished and exported as a single stereo file. The mastering engineer ensures the song is at a competitive volume, has a consistent tonal balance, and will translate well across all playback systems, from iPhone speakers to club PAs.
Think of it like building a house: Mixing is installing the plumbing, wiring, and painting the walls. Mastering is the final inspection and the landscaping that makes it ready for the market. You cannot master a song until the mix is 100% 'locked.'
If you're on a budget, prioritize a great mix over a high-end master. A good mastering engineer can fix a slightly flawed mix, but even the best mastering in the world can't fix a mix where the vocals are too quiet or the bass is out of control.