Can i get some help understanding id="main"

https://codepen.io/shaddehart509/pen/WNoMmrK

An id of “main” may be placed on the <main> element . For example <main id="main">. Then remember to always type # before your id name in your CSS. For example #main{ insert css properties here }

An id is an attribute. So an id="main" means to set an element with an attribute with that.
For instance something like this;
<div id="main">...</div>