A favicon is a small (16 by 16 pixel) icon that identifies a website and shows in the browser tab.
For most WordPress themes, you can add favicon as well as the site icon in the customizer in the “Site Identity” section. If not, you can add a favicon by uploading a square 16 x 16 pixel image to the root of your website (public_html in most WordPress installs) called favicon.ico.
A favicon (/ˈfæv.ɪˌkɒn/; short for favorite icon), also known as a shortcut icon, website icon, tab icon, URL icon, or bookmark icon, is a file containing one or more small icons, associated with a particular website or web page. A web designer can create such an icon and upload it to a website (or web page) by several means, and graphical web browsers will then make use of it. Browsers that provide favicon support typically display a page's favicon in the browser's address bar (sometimes in the history as well) and next to the page's name in a list of bookmarks. Browsers that support a tabbed document interface typically show a page's favicon next to the page's title on the tab, and site-specific browsers use the favicon as a desktop icon.