When a Dom (dominant) collars with a sub (submissive), it means the Sub is related to the Dom. The extent to which you want to be submissive in your BDSM relationship is determined by agreed-upon boundaries and the submissive you want to be.
Some Subs, for example, may only wish to wear their BDSM collar in the bedroom. Therefore, a Dom can do the collaring at the beginning of the play session (also known as a scene) to emphasize that she must follow their operating rules from that point forward. The collar is taken out once the scenario is completed.
Other subs may choose a BDSM relationship that lasts 24 hours a day, seven days a week, and requires them to wear a collar at all times. When they have proven to be loyal and capable, their Dom will collar them during a collaring ceremony. The Sub would wear the collar from then on.