Could you explain <div> tag?

Answer 1

In practice it is often used as a container/organizer for other elements on the page, imagine a website as a bunch of squares and rectangles containing other elements that make up the page as a whole, the squares and rectangles are usually denoted as <div> tags.
HTML semantic elements are just more descriptive <div> tags to make the page more human readable.
(CSS Grid or 3rd party Grid systems are much better imo.)

Answer 2

The div tag is a block level element and is used to group related items together. For example, if you were making a contact page with a name field, an email field and a message field you could group them together.