<link rel = "stylesheet"href="style.css"></link>

Question

What does the mean of this line ?? .
Why should I use this line??

Answer 1

The <link> tag is used to define a relationship between an HTML document and an external resources. 
https://www.sololearn.com/discuss/272844/?ref=app

Answer 2

Link tag is used to connect one document with other external document.
Here in this line "rel" means the type of relationship between documents, "href" is the path where document is located.