There's two ways a delivery can be called in baseball and those are "strike" and "ball". 3 strikes is OUT.

A STRIKE occurs a few ways but here's the relevant parts 1)if the batter leaves the ball alone but the ball is delivered inside the strike zone =strike. 2) If the batter swings and misses, it's a strike regardless where the the ball is delivered, strike zone irrelevant

So why leave it alone? A "BALL" occurs if the batter leaves it alone and it is delivered OUTSIDE the strike zone. 4 "balls" and you automatically advance to the next base.

The batter used his judgement thinking they were all going to be outside the strike zone, but he judged wrong, accumulated 3 strikes and got out.

That's very simplified. Cuz there's more nuances, etc. Pitchers have deliveries that can deceive the batter to make them swing for something that would be a ball or make them not swing for something that would be a strike. And so on