<link href="https://fonts.googleapis.com/css?family=Handlee" rel="stylesheet">

Question

In html wht means this, why it is used

Answer 1

The <link> tag defines a link between a document and an external resource.

The <link> tag is used to link to external style sheets.

Answer 2

This is the cdn of Google Fonts API, it import specific fonts by Google, like in your question, it imports Handlee font
The way to use it

Just need to add this inside the head tag
<link href="https://fonts.googleapis.com/css?family=(font name)" rel="stylesheet">

And write the name of the font in css file in font-family property
font-family:"font name"

All Google font
https://fonts.google.com/

Example
https://code.sololearn.com/WjBK25HqXKXo/?ref=app