Four short lines make one simple argument: if you never tell anyone about yourself, no one will ever understand you. The speaker lists what stays hidden, your reasons and your past, and then widens it to "anything about you." The first three lines build up the same word, "nobody," until it starts to sound like a wall.
The twist comes in the last line. "If you ever only speak to nobody" turns "nobody" from a word meaning "no one" into something like a place or a listener. Speaking to nobody is the same as saying nothing at all. The poem's logic is neat and a little circular, which makes it feel like a hard truth.
The speaker sounds like someone giving advice, maybe gently, maybe with some frustration. The mood is quiet and lonely, but there is also a nudge in it: the poem suggests the way out is simply to talk to somebody.