A favicon is a little icon visible on the web browser tab, just before the page title. It is generally a logo with the smaller size.
Here, you can see the favicon,
The size of a favicon is 16x16 since it also gets displayed next to the URL of your site in a browser's address bar. It is visible on a users list of bookmarks and easily helps in recognizing the website from a list of websites.
To add a favicon icon, you need to create an icon, with size 16x16. Also, some websites provide options to create favicon icon from PNG, JPG, or GIF file formats.
Go to favicon-generator.org and add the image, from which you want to create a favicon icon. Upload the image and click Create to generate the favicon icon.