Mastering is the last step toward creating a polished final product.
It wasn't so long ago that this was the exclusive realm of recording royalty and well out of range of us, the commoners.
The need for highly specialized (and expensive) equipment made this a job for the pros.
This task is performed at the end of a project and is a way to bring together all the tracks into one album.
It's important to adjust the relative levels of the various songs that make up a collection.
Also this is the time to optimize the average level of the project so it be as loud as possible and stand out from the competition on TV, Radio, the Web, etc.
The overall flow of the project needs to be considered here too in order to make it all gel together and sound complete.
Which song should be the first one out of the gate?
Do you want to have a slower song give the listeners a break?
Or do you want to keep the energy level high?
The amount to fade-out between the current track and the next, the empty space between tracks, and the overall dynamics are just some of the questions that will be answered in this phase of the recording process.
Most DAW Recording Software will come with these final-touch features included such as compression, EQ, and peak limters.
So chances are you won't need any additional plug-ins or additional software for this part of the project... especially if you're just starting out.
The discussion of how exactly to master your project is a whole other ballgame though.
This gets into more of the artistic side of audio music production and as always, there is no substitute for familiarity with your software and hours spent in the driver's seat.
Remember that sometimes the best edits are the ones that you don't make.
Don't overdo it by tweaking your project excessively.