Irresistible grace means that when God wants to save someone, that person cannot resist it. Imagine if you were holding a big, delicious piece of cake and someone wanted it really badly. No matter how hard you tried to keep it away, if they were really determined, they would find a way to get it. This is kind of what irresistible grace is like, but with God's love and salvation.
Some people believe that God chooses who he wants to save, and those people have no choice in the matter. It's like being picked for a sports team - you don't choose to be picked, the coach chooses you. And once you're chosen, you can't say no. You have to play on the team.
But some people don't like this idea because it seems like it takes away their free will. Others think that God gives people the choice to accept or reject his love, and that everyone has a chance to be saved if they want to be.
Either way, the idea of irresistible grace means that when God wants to save someone, they can't say no. It's like God's love is too powerful for them to resist. It's kind of like being caught in a really strong magnet - no matter how hard you try to pull away, you're still going to be drawn to it.
It's a complicated idea, and people have been arguing about it for a long time. But basically, it's about how we can't resist God's love if he really wants us to be saved.