Okay kiddo, imagine you are playing a game with your friends where you have to build a big tower using blocks. You start off by putting one block on top of another and keep adding more blocks on top until you make a really tall tower.
In music, a block chord is kind of like building a tower too, but with sounds instead of blocks. It's when a bunch of notes are played at the same time, like putting a bunch of blocks on top of each other all at once.
Imagine I have a bunch of different colored blocks and I want to stack them all up in a tower. But instead of doing it one by one, I put all the blocks of the same color together and stack them up all at once. That's what a block chord is like - playing a group of notes together all at once.
Block chords are used a lot in jazz music, where they can create a really full and rich sound. It's kind of like when you have a lot of blocks stacked up really close together - it makes a bigger and stronger tower!