A bare bottom is the exposed or naked buttocks. It is common for the spankee's bottom to be bared for the spanking, or even for the spankee to be spanked totally nude. This is often refered to as a bare bottom spanking.
For disciplinary spanking, the main reason for baring the bottom is to increase the pain, and additionally, to humiliate the spankee and make the punishment more embarassing (especially when others are watching, e.g. classmates). Another aspect is visual feedback: the redness of the bottom, bruises and other signs can indicate how severely the delinquent has been spanked already and when it is time to stop.
For erotic spanking, the spankee's bottom is almost always bared, but here the main reason is for both partners to enjoy the erotic experience of the spankee's bare bottom. When the buttocks have been marked, the bottom may be required to bare the buttocks in the household. Displaying such marks reinforce the spankee's humiliation.
It is also not uncommon, to enhance the procedure and the overall spanking experience, to begin the spanking on the fully clothed bottom (as a warm-up spanking), then remove one layer of clothing, continuing the spanking on the panties-, or briefs- or boxer briefs-clad bottom, and when sufficiently done, finish the spanking with a bare bottom spanking.
In addition to spanking on the bare bottom, a child or an adult spankee may be instructed to remain in a bare bottom condition for a time, as a form of humiliation and discipline. This may occur after a spanking, when the results of the spanking are displayed, or as an alternative to a spanking. This may be combined with corner time.