Ok i'm going to make this simple incase you know nothing about sites.
Lets say my website is a lake, when you go to my website all you are seeing is the surface of my "lake", but underneath that surface there is a bunch of coding called HTML, it stands for Hyper Text Markup Language. It works something like this
HTML:
TO THE RIGHT IS THE DESCRIPTION- TO THE LEFT IS THE TAG
bold text = <b>
Italic text= <i>
Underlined text= <u>
So if i used the "tag" for bold it would display bold text on my site.
Each thing being displayed has its own tag, Images, text, links etc.
Sorry for explaining this if you already know html lol.
But if you don't know HTML you can learn it
Here