A favicon (or “favorite icon”) is the 16 pixel square image that precedes the website title in browser window tabs (see illustration). Nowadays, favicons are used for tile and touch icons, iPhones and Android/Chrome apps, so they need to be larger (WordPress likes a 512 x 512 pixel image) so that they can be distributed correctly on different devices and apps. Thumbnail logo images, of which browser tab favicons are the smallest, are not legible, so we recommend a simplified logo, or an initial or monogram in the brand color(s). This applies to avatars and social media profile images as well.
You can see how spare our logo looks here at 75 pixels, but it looks appropriate in a browser tab or as an app image. Favicons have become hilariously complicated over the years, but the WordPress soultion has at made favicons easy to implement for millions of website admins.