Audio mastering is also an important step in creating a finished song or album.
Generally, individual song mastering includes frequency adjustments / balancing and layers of compression / limiting to control dynamics and optimize volume.
Album mastering is the process of turning a collection of songs into a final product by making them sound like they belong together in tone, volume and timing. In essence it is the process of smoothing over any level and spectral imbalances and to present the final recorded project for its intended media form.
Another result of mastering is achieving the overall optimum volume level. Traditionally, the industry as a whole tends to set the average level of a song at the highest possible level without distortion.