When you hear the phrase 24/7/365 you know that it means all the time, or in retail speak that they never close. Used in the Dominant/submissive context it means something similar.
A 24/7 D/s (or M/s) relationship is based on total power exchange. This is when one person surrenders total control to another for an indefinite period of time. In a relationship like this the power exchange is super-imposed over the other roles that these people may assume; such as parent or spouse. One person is always dominant and the other is always submissive to them. Sometimes also referred to as Lifestyle D/s.
While 24/7 Dominant/submissive relationships imply that the couple live together in this new technology age that is not always the case. As long as the roles are maintained there can be a total power exchange from a great distance. It is also necessary to define if you are 24/7 or not in the case of D/s because unlike other roles like parent or spouse, it is possible to drop the D/s part of the relationship if that is agreed and if you only participate for definite periods of time.