Imagine you have blocks of different colors, like red, blue, and yellow. You want to stack them up, but you have to follow some rules. You can only put a block of the same color on top of another block of the same color. And you can't leave any spaces between the blocks, they have to be touching.
The Castelnuovo-Mumford regularity is like a measuring stick for these blocks. It tells you how many layers you can stack up before you have to start using a different color block or leave empty space.
So, if your blocks are very organized and you can stack many of the same colored blocks on top of each other, your regularity number will be low. But if you have to switch colors or leave spaces very quickly, your regularity number will be higher.
This concept is used in math to study algebraic shapes called varieties. Instead of blocks, you have equations that define the shape. The regularity number tells you how many equations you need to use before you have to start using more complicated equations or leave gaps.
It may seem complicated, but remember: it's like stacking blocks!