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.
Other Definitions Online
on Submissive Guide
What else do you have to include about 24/7 D/s (M/s)?