Answer 1

Usually for sending datas about your site to search engines and the browser.

Examples:

You can use this code to change description of your website (used by search engines):
<meta name="description" content="My website descriptions here..." />

You can write this code to change text of the browser tab:
<title>Page Title</title>

You can connect to a CSS or JavaScript file; Connect to your CSS:
<link rel="stylesheet" href="styleUrl.css" />

And other works...
Just remember that the codes in the head are invisible.