So, you know how sometimes we like to express our feelings and thoughts in words that rhyme or have a rhythm to them? That's what poets do! And sometimes, lots of poets get together to compete against each other to show who can use words in the coolest or most powerful way. This is called a poetry slam.
Now, imagine that the whole country had these poetry slams and the best poets from each place got to compete against each other. That's what a national poetry slam is! It's a big competition where poets from all over the country get together to see who is the best poet of them all.
They perform their poems in front of judges who decide who is the winner based on things like how they use words, how they sound when they speak their poem, and how much emotion they put into it. It's like a big show where people get to cheer for their favorite poets and see who is the best at using words to tell a story or express a feeling.