How to make a block of text with a specific background color?

I am new to this whole thing. I want to have a part in my website which is basically a text overlaid on a rectangle of a specific color. Can I do that? How? The learning resources are pretty useless for matters like this.

If I'm understanding you correctly, this is just rudimentary html/css. I suggest you start by learning how to build a webpage if that's what you're interested in. There's plenty of great resources online you'll find by searching.

The documentation won't tell you how to do this – it's documentation for Grav, not basic web design.