In the most basic words, Tempo means doing a lot of stuff that affects the board (so mostly playing & removing minions) for as little mana as possible. And Rogue has always been an expert when it comes to that with cards like Backstab and Preparation.
Tempo Rogue is a deck that takes advantage of this kind of play style – it’s a rather aggressive (but not Aggro) build that plays many swing cards, such as Sap (you can get rid of a big minion temporarily for just 2 mana) or SI:7 Agent (it’s a body + removal in one).
So far, Tempo Rogue is one of the most powerful archetypes in Rise of Shadows. Given that it didn’t lost much of its power and got a few new, great tools (EVIL Miscreant, Vendetta, Waggle Pick, Heistbaron Togwaggle etc.), it’s a big part of the expansion’s early meta.