It defines location of the file in website. The file may be image, web page ,style sheet or java script.
<img src="https://codingpk.com/images/apple.jpg" alt="Apple" height="200" width="250">
This specify full URL address of file.
<img src="https://codingpk.com/images/natural.jpg" alt="Apple" height="200" width="250">
This specify to file that is related to current page.
<img src="/images/apple.jpg" alt="Image" style="width:300px">
This path points images folder in current folder.
<img src="pic.jpg">
It specify pic.jpg is located in same folder of current page.
<img src="images/pic.jpg">
It specify pic.jpg is located in images folder of current folder.
<img src="/images/pic.jpg">
It specify pic.jpg is located in images folder at the root of current web.
<img src="../pic.jpg">
It specify pic.jpg is located in one folder up of current folder.
<img src="../../pic.jpg">
It specify pic.jpg is located in two folders up of current folder.