In BDSM a switch is a person who participates in BDSM activities either as a Dominant or a submissive, also referred to as a top and bottom. Switches tend to enjoy both roles, sometimes equally, sometimes one more than the other. They may either switch within a relationship or indeed feel they can only switch with other people outside of their relationship. It is rare that a switch will switch during play as this can lead to confusion on both sides and may lead to uncertainty. This would be something that you would discuss and agree in advance, as naturally you need to feel assured, safe and secure in a scene or role.
BDSM is not only about being Dominant or submissive. Some enjoy inflicting or receiving pain, or other types of stimulation but do not wish to have the psychological power or control element of say a Master or Mistress has over their slave or sub. Quite often people that enjoy this side of BDSM will be switches, but again this is not always the case. We are all individuals!
Within the BDSM community some frown on a switch and feel that they should make up their mind and be one or the other. Views differ, however if you are a bisexual lady or man you are not asked to "make up you mind". Others also feel that as a switch you can experience both sides and that it gives them greater insight into the dynamics of the power exchange and therefore have a better understanding of how both sides feel, which can lead to an even more fulfilling role.