It's been an oft-discussed question on StackOverflow what this means:
As of December 2014, Paul Irish's blog on protocol-relative URLs says:
2014.12.17: Now that SSL is encouraged for everyone and doesn’t have performance concerns, this technique is now an anti-pattern. If the asset you need is available on SSL, then always use the
Unless you have specific performance concerns (such as the slow mobile network mentioned in Zakjan's answer) you should use
https:// to protect your users.