Under Protection means that the submissive is under the protection of that particular Dominant, protection gives the submissive the security of knowing she is safe when out and about, as she is under protection. She is literally unapproachable without the knowledge of the protecting Dominant.
Usually, it is a Dominant of high standing in the community that is approached for, or offers protection. There is no time limit on a collar of protection, it is there for as long as it is needed.