Basically, HTML is a type of code that act as a sort of "action word", as I like to think of it.
It's like playing video games. Hit "up" on the control and the character will jump. Hit left-right-down and the character will...I dunno, look around and crouch.
With computers, you put in a code (aka HTML) to make your text a different font, change your background color, make tables, etc. It's the easiest way (imo) to customize your website.
: It's a site for beginners