The header tag, H1, is a very, and fairly simple tag, it just,

makes things bigger.

I don't really need to go too in-depth with it, as much as I should. The H1 tag makes things bigger, if you need to. Like I said, it's a header tag, so, it's simple! Use it for a header! There's also varying sizes, h2 through h6. Each one gets

smaller,

smaller,

smaller,

smaller,
and smaller!

That's really it for H1, and its smaller variants. Below is a graph showing this as well: