Push Record is not a mastering house, but offers a light mastering service suitable for many people.

After mixing down into one stereo track, the mastering process further produces the sound, making it both smoother and louder. The dynamic range (quietest to loudest parts) of the song must be reduced, whilst boosting its overall level. The volume dynamic you are used to hearing from your stereo is significantly less than the dynamics produced from live instruments. Even after compression during the mixing stage, the overall file will still have too great a dynamic range. Compressing during mastering also smooths the song out, glues the instruments together, and allows for the overall “loudness” to be increased.
The track should also be listened to on a range of devices (for example laptop speakers bring out some middle and high frequencies that can highlight problems with the mix that need resolving).

There are two essential things to bear in mind when mastering, firstly to make sure the mix between bass and treble is well balanced since the range of music devices available can highlight issues at either end of the spectrum. Secondly the controlling the compression used to give the song its “loudness” is important as too much compression can add noise that becomes audible as the song is converted to mp3 and sites like Soundcloud can make it worse.