Read quickly, the three lines sound like a riddle. The last words you say will be full of regret, and the reason is that you won't say them at all. The twist is in that last line. Regret usually comes from something we did say, but here it comes from silence.
The speaker seems to be thinking about the things people hold back: an apology, an "I love you," a truth they put off. Because death or a final parting often comes without warning, those words never get their chance. The poem suggests that our real last words may be the ones we meant to say and never did.
The mood is quiet, heavy, and a little unsettling. The short lines and plain wording feel like a sigh, and the abrupt ending leaves a gap where the words should be. It reads as a gentle warning: say what matters while you still can.