A favicon is the small image that helps identify a website in browser tabs, bookmarks, and similar places. It is like a tiny badge on a folder that helps you spot it quickly.
The icon often uses a simplified version of a site's logo because it must remain recognizable at a very small size. Browsers may show it beside the page title.
Keep the design simple and provide the icon in a format the site supports. A favicon is a small detail, but it helps people find a site again among many open tabs.