Okay kiddo, let me explain the iron cross to you like you're 5 years old. The iron cross is a gymnastics move where you lift your body up in the air and hold it straight like a board. It's called the iron cross because it looks like a cross made of iron (but don't worry, you don't actually have to be made of iron to do it!).
To do the iron cross, you need to be very strong and have good balance. You usually do it on a set of rings hanging from the ceiling. You grab onto the rings with your hands and lift your body up in the air until your arms are straight out to the sides, like you're making a big "T" with your body.
Once you're in the iron cross position, you have to use all of your strength to hold yourself up there. It's like doing a really hard push-up, but even harder because you're holding yourself up in the air instead of on the ground.
Not many people can do the iron cross, because it takes a lot of practice and hard work to build the strength and coordination you need. But if you keep practicing and working hard, you might be able to do it someday too!