Favicon & Touch Icon
A favicon is the small symbol that represents a website in browser tabs, bookmarks and search results, while a touch icon is a larger version of it used when a page is saved to a mobile home screen.
In practice
Providing several icon sizes in the HTML head of the website or in a manifest file makes sure the icon is rendered sharply on different devices and in different contexts, instead of being scaled up unattractively from a single small image. For display in Google search results the favicon has to be square (a 1:1 aspect ratio) and at least 8 x 8 pixels; Google recommends more than 48 x 48 pixels so that it looks good on every surface.