Random tech question: HTTP v. HTTPS

[PackerFanatic]

We are including a hosted JS library on our site and the company we are using told us to include it like this...

src=//libs.site.com...

According to them, the SRC knows to just use whatever the page is using if you use //, instead of explicitly having to specify HTTP or HTTPS.

Has anyone ever heard of this? Our site didn't seem to like // much (as it was throwing some security errors on our secure cart pages), but according to them, it should be working.