A favicon is a little icon that appears in the browser address bar (also in the browser tab and bookmarks). Favicons are mostly 16x16 pixel square. Favicon is a .ico file (although Firefox also supports PNG images, but most of the browsers only support .ico format). You can create your own favicon using a graphics tool or you can use the following online tools to create a new favicon.
After creating an icon for you website, upload it to the root directory and use the following tag to add this icon to your website
<link rel="shortcut icon" href="favicon.ico">