A load-bearing person is the one the structure quietly depends on. Pull them and something you did not realize was holding weight comes down.
Every team has one, and most teams find out who it is the week that person leaves. Not always the most senior, the loudest, or the one in the title; usually the one who absorbs the things no process covers. Their work is invisible precisely because nothing breaks while they are there.
It is a compliment and a warning at once. A capability that lives in one person is a capability the organization does not actually own yet. Find them before the exit interview does.