StaFi is a combined abbreviation for Staking + Finance. The contract class consists of many stake-out contracts and the application layer is mainly the rTokens transaction platform. If you use PoS tokens via StaFi, you will receive a corresponding amount of rToken. For example: if you put Atom you can get a rAtom. The staking out of XTZ receives rXTZ. If you own rToken, you have the right to change the corresponding number of tokens and receive the stake reward. Alternatively, it is possible to trade rToken instead of setting tokens.