A 'Favicon' (short for Favorite icon), also known as a shortcut icon, is the little image seen next to your website in your browse's tabs as well as in some other places such as the bookmarks folder.
In the image above this is looks like a small 'C' in a blue square - the standard Create favicon.
You can't use any image for this, you need a really small 16 X 16px ico file. There are lots of free tools online you can use to create your own one.
The following browser-based tool, allows you to either upload an image and scale it down to the right size for the favicon, or draw your own:
Then, once you are finished, you click "Download Favicon" which is below the "preview" to save it to your computer.
Please note that the file name of your favicon must be "favicon.ico" exactly in order to work.
You will then need to publish your website to see the changes live.
If your icon is taking a long time to appear, you may need to clear your browsers cache. You can find some more info on this guide 'How can I clear my browsers cache?'