Mastering is mostly a volume/dynamics entity. Plus some EQ and frequency based compression. A song that is just a combination of the parts is only half the way there. In the recording process you make all your tracks, but when you put them together, everything is competing for the same space in your ear. The right amount of compression and multiband limiting makes it so that the actual audible volume of each component of your mix can punch through more clearly because when it comes out, the compressor dips the rest of the program down.
Like drums for example. You can set the drum volume slightly lower than perfect, knowing that the final mastering will bring down the rest of the program for the split second of each drum hit. The drums will be very audible but the melodic portion will be more of a "wall of sound". Or you can mix the drums hot so they peak the limiter, then the mix will seem more dynamic, even if its compressed heavily. Mastering also makes all your songs balanced with eachother, so the listener doesn't have to turn the volume up for certain songs and down for others.
Sometimes the role of EQ is to make each track "feel" similar aurally. Other times EQ is used to manipulate the compressor/limiter into giving more dynamic range to a certain frequency area, to focus on an instrument, or an overall "sound" to your mix. Like Adult Contemporary music often has weak bass, fizzy treble, and sibilant vocals. Hip Hop has thunderous bass and sometimes not much else. Metal has gone through a huge evolution in 30 years, from "transistor radio blasting midrange" to a wide, full frequency program. Most commercial artists have a continuity to their sound, even if the songs are very different sounding. Some of that is the mastering.
If you're trying to make a pro recording to sell or demo, it should be mastered. You won't believe the difference. So either learn a lot about it or hire it out. If it's a serious project then hire it out. If its just a fun practice band then have fun learning.