Okay kiddo, have you ever played tic-tac-toe? Imagine you have a big square and you draw a line from the top left corner to the bottom right corner. Then you draw another line from the top right corner to the bottom left corner. That's kind of what a cross fleury looks like! It's a symbol that's made up of two lines that cross each other diagonally to create a cross shape.
The word 'fleury' actually comes from the French word for flower, and if you look closely, you might notice that the tips of the lines on the cross fleury look a little bit like petals on a flower. This is because they're usually curved or pointed in a way that makes them look a little bit like they're blooming!
The cross fleury has been used in lots of different ways throughout history. Sometimes it's been used as a religious symbol, to represent the cross that Jesus was crucified on. Other times it's been used as a symbol of a certain family or group of people. It's also been used as a decorative element on things like coats of arms, shields, and flags.
So basically, a cross fleury is just a fancy way of drawing a cross with some pretty flower-like detailing on the tips of the lines!