Favicon is an icon for your web page, it is visible in many places such as browser tab header:
Unfortunately there is no standard size of favicon and each browser expects something else. What is even worse, mobile vs PC browser version might slightly differ in that also.
Fortunately there is place like that:
https://realfavicongenerator.net
Where you can generate different versions of your favicon together with scripts necessary for its usage.
There is also nice service which can verify you page in order to understand whether all favicons have been properly generated:
https://realfavicongenerator.net/favicon_checker