What is mastering?

When your song is finished being mixed, before publicly releasing it, you take it through a series of steps to enhance it called mastering. Mastering is an all-encompassing term for postproduction activities that include the following:

• Making the song sound consistent with other songs in the album
• Editing out flaws
• Equalization to ensure a well-balanced frequency range
• Compression to balance dynamic range
• Stereo width enhancement
• Limiting to raise the overall volume of the mix
• Listening to the audio on different devices and ensuring that there is a consistent quality of sound heard across them
• Any other adjustments necessary to prepare the music for distribution

You should always master your song before publicly releasing it. Mastering should always end up making your music sound better than it did before mastering.

Can you master music yourself?

I personally say yes, or you should at least learn enough about mastering...