Fusion Drive is a utility that combines an SSD and traditional hard drive into a single storage space. Your Mac then automatically manages where data is stored, moving files and apps between the SSD and HDD depending on how often they're used. It's a great compromise between SSD's high speed and HDD's large capacity.
While Fusion Drive is officially only supported on an iMac or Mac mini purchased with an SSD and HDD combination, it can be set up on any supported Mac. Macworld published a useful guide back in 2012 on how to do this and after a few minutes in Terminal, you'll have a fully working Fusion Drive set up and ready to use.