Mastering is an often misunderstood part of the process of producing music. Either people confuse it with mixing or rely on it too much to fix a bad mix. While mixing can be seen as the balancing of the individual elements of the music, mastering should be viewed as controlling the tonal, dynamic and spatial aspects of the overall sound. In mixing you may think that the snare drum needs more attack and the acoustic guitar is lacking in it's fundamentals on a certain chord. This can be easily fixed as at the mixing stage you have access to the individual instruments and the processing that is applied to each. Fixes such as this should be made during mixing. When it comes to mastering however, changes that are made are most likely to be applied to the track as a whole, to a waveform that is the sum of all the individual instruments.
The processing involved in mastering will most likely contain equalisation, multiband and single band dynamic control (compression, limiting, expansion etc...) and possibly spatial enhancement (reverb, stereo width, phase alteration...). The processing is used to enhance the dymanics and tonality of the music and ensure consistent sound and levels across multiple tracks if required.
It is not always the case that processing applied at the mastering stage is applied to the overall waveform. Some engineers prefer to be given 'stems', or groups of instruments to work with to allow slightly more control. For example the mastering engineer may be given a stem containing all the drum tracks, a guitar stem and a vocal stem. This would allow the engineer to control certain low frequency problems in the drum tracks without affecting the sound of the guitars as well.