Any URL specifies its location on a computer network and a mechanism for retrieving it. URLs consist of two mandatory fields, (a) type of resource such as
https for webpages,
ftps for file transfers,
mailto for emails, etc., and (b) hostname, as well as of optional (c) one or more submomains and (d) a file name, if any.
According to the Business Insider, Sir Timothy John Berners-Lee once said about the double slash in URLs,
Really, if you think about it, it doesn't need the //. I could have designed it not to have the //.