Mixing and Mastering/Introduction
Audio mixing, in the broadest sense, is the craft of taking various audio streams and balancing them to produce a cohesive whole. This can apply to many different media, including music, movies, television, video games, audio books, and anything else which has an audio component. In each of these different art forms, you can have anything from individual instrument tracks, sound effects, spoken word, etcetera. A well mixed audio track will emphasize the element of itself that is most important to hear when it is played. For instance, in a movie, while the musical score is important, the most important track will usually be the dialogue. If the score is making it difficult to hear what the characters are saying, it has been poorly mixed.
By contrast, mastering is the process of taking already mixed audio streams and preparing them for publication. The goals of mastering can change drastically depending on which kind of media you are publishing and on what format it is stored. One common application of a mastering process is to take several songs which are to be compiled into an album and ensure that each track is perceived as the same volume and that all tracks have similar sound profiles; that is to say, to ensure they sound like a unified product rather than a random collage. Remasters often are commissioned when an older work is being published again on a new medium, for example, an old vinyl album being released in a digital format. The sonic capabilities of different media are quickly becoming historicized, as everything today is being published in digital format. But interesting changes to digital mastering are happening as streaming services being doing their own automated processes to any files uploaded on their services.
Both mixing and mastering use similar techniques and tools (equalization, compression, saturation, etcetera), but they are different skill sets. Learning one will teach you important things about the other, but like any other art form, the only thing that will improve your skills is practice in a specific domain.