Sia uses a cryptocurrency called the Siacoin (SC). The developers mined the first 100 blocks or so before releasing the code and miner to the public. Other than these first blocks, there was no premine for Siacoins. The initial block reward was 300,000 siacoins. Each block reward after that is one siacoin smaller than the previous block reward (299,999, then 299,998, and so on). When the block reward reaches 30,000, all remaining blocks will give a 30,000 siacoin reward. The block time is 10 minutes. Each Siacoin is composed of 10^24 indivisible units (hastings).
The most important features of Sia can only be accessed by using Siacoins. All storage contracts and all Sia payment channels require owning Siacoins. This requirement means that as Sia grows in usage, so too will demand for Siacoins. As demand grows, the price will increase. If Sia is being used for billions of dollars of storage, billions of dollars of Siacoins will be required. The value of the Siacoin is inextricably tied to the amount of storage in use on the Sia network.
(edited from source)
A Hasting is the smallest unit of currency in Sia, equal to 0.00000000000000000000001 siacoin. The reason for such a small unit is because Sia often relies on very small micro-payments, way smaller than Bitcoin could possibly handle. These micro-payments are necessary because storage prices are calculated as Hastings per Byte per block. Since a block is 10 minutes, and single bytes are very cheap you end up with a very small number.