A backlog is the running list of candidate work items a PM keeps and continually reprioritizes. Prioritization is the act of ranking backlog items so the team knows which ones "can't wait" versus which can be deferred.
A useful discipline: every addition should result in a deletion. A backlog that only grows isn't being curated — treat its shrinking size, not its length, as the signal of progress. This connects to Work-in-Progress Limits for Strategic Focus (limiting unbounded accumulation on the execution side) and feeds directly into the Roadmap (Feature/Project List) once items are selected.