There are a lot of terms used in the mixing process that don’t mean anything to the average person. We audio types often forget that. Here are some simple explanations for clarification.
Mixing is the process of creating a stereo mix of all the separate tracks, and applying equalization (EQ), effects and compression (dynamics). A trained ear will know where to place different instruments in the audio spectrum, and how to treat each instrument to get the best overall sound.
Stems refer to the individual tracks of a mix. If you wanted to record drums with us, and then bring them to another studio, you would probably need the stems. This means the kick drum as its own stereo file, the snare drum as another, each guitar track, etc. Another engineer will be able to drop these tracks into their own recording program, and work with each instrument to mix them to their own liking. You typically don’t need this unless you’re planning to work in multiple studios or do some sort of remix in the future.
Mastering is the process of achieving perceived loudness through compression and limiting. When you listen to your rough mixes in your car or computer, you’ll notice they play a lot quieter than the music you normally listen to. This is because they have not yet been mastered. Mastering makes the quieter parts of the song louder, without making the louder parts too loud. This can be a tricky area, because loudness can ruin a song if approached the wrong way. If you wrote a gentle folk song, with soft verses and a LOUD chorus, you probably don’t want the whole song blasting out of your car speakers. You want dynamics — the range between soft and loud sounds. A good mastering engineer will know how to approach your songs, based on the genre, the overall sound, and how the music is going to be released (internet, CD, vinyl, etc.).
If you’re looking to save some money, https://www.landr.com/ is a great resource for mastering! You can drag and drop audio files directly onto the website, and get a mastered version back in minutes! The only thing to keep in mind is that a computer is doing the work for you. Human ears can’t be topped. Yet.