What does 1 Samuel 3:12 mean?

The Lord is revealing a prophecy to Samuel in the middle of the night. This is the first time the Lord has ever revealed Himself in a personal way to Samuel, who is still just a "boy" (1 Samuel 3:1, 7).

To make it even heavier, this is no easy prophecy. For one thing, the Lord has said that the news of what He is about to describe to Samuel will "make the [ears] of everyone who hears it" ring as if they've been struck (1 Samuel 3:11). In other words, this is a national prophecy with enormous consequences. Second, it's bad news about Samuel's mentor and guardian: Eli, the priest of Israel.

The Lord tells Samuel that on the day He does this thing, He will do everything He has spoken against Eli and his house, meaning his descendants. The Lord will bring it all to pass from start to finish. This will not be a partial fulfillment. God will leave none of the judgment undone. It's unknown whether Samuel or anyone else knew anything about the prior revelation from an unnamed prophet. That messenger of the Lord predicted that judgment was coming against Eli's household. What the Lord says to Samuel here is a confirmation of that prophecy (1 Samuel 2:27–36).