So you know how sometimes you can see little specks or bits of dirt on your shirt or on your toys? Well, diamonds can have little specks or marks like that too, but they're called "inclusions" or "blemishes" and they can make the diamond look less shiny and sparkly.
When people talk about a diamond's clarity, they're talking about how many of these little marks there are and how easy they are to see. A really clear diamond doesn't have many marks at all, and they're hard to see even if you look really closely.
Some diamonds have more marks or inclusions, but they can still be really pretty! It just depends on what kind of look you like best. Some people like a diamond that's really clear and sparkly, while others like one that has more character and uniqueness to it.
Diamonds are graded on a scale from "flawless" (that means NO marks at all!), to "included" (that means LOTS of marks). So when you're looking at diamonds, you can ask the person helping you what the clarity is, and they can show you different options based on what you're looking for.