The favicon is the small icon in a browser tab, in bookmarks and on a phone home screen. It is tiny, but it is one of the first things that makes a site look finished rather than unfinished. Browsers and devices each expect their own sizes, which is tedious to produce by hand. This tool takes one image and generates the icon files a website needs.
A simple, square, high contrast mark. Detailed photos become unreadable at sixteen pixels.
A small icon for browser tabs and larger ones for phone home screens and bookmarks, which is what the generator produces.
Upload the files to your site and link to them from the head of your pages.
Browsers cache favicons aggressively. Clearing the cache or a hard refresh usually reveals the new one.