What is mastering?

Mastering, technically speaking, is preparing audio for a release format.   Playing music in the car is a release format, playing music in a club is a release format, uploading a song to Youtube is a release format, burning a CD is a release format, and listening to vinyl is a release format.  Plenty of formats!

Those all have different file formats, levels of quality and detail, and sonic characteristics that are different than whatever you've been listening to this whole time while working on your project.  iTunes is different than Spotify, which is different than Youtube, which is different than Lossless etc...  it can be an art just reproducing exactly what you've been hearing so far to the end listener, without even trying to change a thing. 

Avoiding changing file formats and sticking with the highest resolution sources until the very end is one thing that can be done to get a better product.