Favicons are the little icons used by browsers:
However, they can be tricky to get right. Different platforms require different size icons so often it's best to use some kind of generator instead of making all the links manually. The images links are defined in a
manifest.json file. Research the best way to do this with Ruby on Rails and implement it.
This is a site I found that may work - https://realfavicongenerator.net/favicon/ruby_on_rails#.XEjegfwnZ24