This is a technique that will allow you to add punch and clarity to your mixes without paying for someone to master them for you. The way that this is done is by separating the middle of the stereo signal from the sides and EQing them separately so that you take advantage of the whole stereo range. While nothing can take the place of professional mastering, it is nice to be able to play your tracks out without worrying if they are going to get clobbered next to a professionally mastered track.
If you are interested in getting more Mixing and Mastering tips, check out the Ableton Cookbook Live Course! In addition to a module on Mixing and Mastering, there’s 7 modules of Ableton-ey goodness.