What is Sia? Basically storage — think dropbox, google drive, etc. The shtick is that it is decentralized, encrypted, peer to peer storage — although, no one party is holding the information you are storing — it is encrypted, multiplied, chopped into little bits, and then all those little bits are sent around to a bunch of different hosts. When you want access to your file, the appropriate bits are recompiled and you get your file. Think of your file as being in a T-1000 type state.
By building the framework and outsourcing the storage to anyone with a hard drive and an internet connection Sia has reduced overhead dramatically and can charge a much lower fee. They multiply the bits in case a node holding a piece of your file happens to be offline (even though they are incentivized not to be).