The DBD ID is the code Dead by Daylight uses to identify an account across platforms. It appears under your name in-game and looks like Name#abc123.
It is how players find each other to set up a custom lobby, which is what every competitive match is actually played in - the platform organises and records the match, but the trial itself happens in a private game the players create.
DBD Arena requires one on every account for that reason, and shows it inside match rooms so the host can invite the right person without asking.