Clipping
That dreaded red level zone of clipping becomes apparent when there is a volume level higher than 0 decibels or 0db. It is where the overall volume approaches distortion an goes over the standard allowed limit for reproduction on cds or vinyl.
Usually clipping is always indicated by a red light on a meter that wont go out until adjustments are made. The changes necessary to get a mix where no sounds are going over 0db an still remain at a decent level to be heard is done in the mixing stages before a mastering project. Now there are mastering houses that offer the service of mixing and mastering, but if you mix a song yourself, it's best you make sure you did not leave anything clipping.
On the other hand, mastering a piece of music that has some clipping may or may not be too difficult to handle. On a professional stand however, mastering anything that in exceeding or clipping over 0db is a not good. The mastering engineer usually will reject a project and send the artist away to get the song remixed to give it some headroom for the mastering process. Any attempts of mastering a song in a clipping condition will result in distortion as well many other imperfections in the final master.