After the introduction of the magnetic tape in the late 1940s, mastering became a very specialized and highly prized skill. It was often conducted in studios by engineers wearing white lab coats and became known as something strange and mysterious, incomprehensible by most people.

We feel the urge to demystify. Mastering is simply the last step in preparing your recorded and mixed audio content for duplication, replication or digital downloading. Proper mastering will give your tracks a cohesive feel and "gloss" that is a signature of higher quality commercial releases. It ensures that the volume is consistent from track to track, and deals with any potential frequency problems. And yes, mastering is the process of making your songs "louder" in simple terms.

Unlike recording and mixing, mastering is less subjective. There is a level of depth and clarity that can be achieved. The goal is to elevate the mixed content to the highest possible level. Mastering should be crafted by an experienced engineer with well trained ears and the technical ability to bring out the best in your audio.

Mastering is absolutely necessary if you want what's best for your project.

You can read further about mastering on Wikipedia here.