It means that a secure connection exists between the client and the host. Everything going between the two is encrypted and a secure tunnel exists. There are obvious benefits to it when money is being transacted.
As for why sites like youtube do it... you got me. It DOES slow things down. Maybe they want to stop interlopers from seeing what you're up to.
*** CORRECTION *** I've done a little reading and it is actually faster - because the websites involved are already certified as secure. I didn't know that. That explains why youtube likes it.
Gold is $1,581/oz today. When it hits $2,000, it will be up 26.5%. Let's see how long that takes. - De 3/11/2013 - ANSWER: 7 Years, 5 Months