Okay kiddo, do you know what a pencil is? Good! So, a pencil is made of lead inside a wooden case. Now, pencils come in different hardness levels. This means that some pencils are harder than others.
You see, the lead in the pencil is made of graphite and clay. Depending on how much clay is in the lead, the pencil can be harder or darker. The harder the pencil, the more clay it has.
When you use a hard pencil, it's easier to write lightly and not smudge the paper. But if you want to make darker lines, you would use a softer pencil with less clay in the lead.
So, if you want to write lightly or draw precise lines, you would use a hard pencil like an H or 2H. But if you want to shade or draw darker lines, you would use a softer pencil like a B or 2B.
And that is how pencil hardness works! Pretty cool, huh?