Why are there no slashes in URLs beginning mailto:, two slashes in URLs beginning http:// or ftp://, and three slashes in URLs beginning file:/// ?

What do those slashes mean? What are they FOR?

A mailto is not a URL but a HTML tag. The 2 slashes after http and ftp separate the protocol from the actual address. I don't believe 3 slashes is necessary after file. If its there it may be to indicate a relative address.