Favicon was first used in march 1999, when Microsoftft released Internet Explorer 5 that time it was used to make tabs a favorites (bookmarks).

Favicon is a small square 16×16 pixels image, Basically It is used on tab at the web browsers to show a graphical representation. This approaches only works in HTML or XHTML.

To add fav icon you need to add below link in head tag in your page-

<link rel="shortcut icon" type="image/x-icon" href="http://yourwebsite.com/favicon.ico">

Fav icon icon format should be .ico but modern browser also support .png format

There are many online tools to make favicon

But ios operating system have some other scenario, Apple devices with the iOS operating system after version 1.1.3 and in android devices, you can customize the fav icon. You can make rounded corner and without reflective shine, to do this you have to add below line

Without reflective shine

<link rel="apple-touch-icon-precomposed" href="somepath/image.png" />

With rounded corners, added by iOS

<link rel="apple-touch-icon" href="somepath/image.png" />