The Beltrami identity is a fancy way of saying that some things always stay the same no matter what you do.
Imagine you have a pizza, and you want to cut it into slices. You can cut it into two slices, four slices, or even eight slices. No matter how many slices you cut the pizza into, it's still going to have the same amount of pizza. The Beltrami identity is kind of like that.
Scientists use the Beltrami identity in something called fluid dynamics, which is a way to study how fluids (like water or air) move around. They use the Beltrami identity to figure out how the fluid is spinning and twisting around.
One way they do this is by looking at something called a vortex. A vortex is kind of like a tornado in the water or air. When a vortex forms, the fluid starts to spin around like a whirlpool. The Beltrami identity says that no matter how fast the vortex is spinning, some things stay the same.
For example, let's say you have a cup of water, and you start spinning it in a circle. The water in the middle will start to spin faster than the water on the outside. But the Beltrami identity says that the amount of water in the cup and the area it's spinning in will always stay the same, no matter how fast you're spinning it.
So that's the Beltrami identity in a nutshell! It's a way scientists use to figure out how fluids are moving around, and it helps them understand things like vortices and whirlpools.