xSUSHI is a token, similar to our SLP tokens, that you receive in exchange for staking SUSHI tokens in the Sushibar. While holding the token, it will appreciate in value, as fees from our exchange platform are "served to the Sushibar." The xSUSHI token is always worth more than a regular SUSHI token, because xSUSHI accrues value from platform fees. When users make trades on the SushiSwap exchange a 0.3% fee is charged. 0.05% of this fee is added to the SushiBar pool in the form of LP tokens. When the rewards contract is called (minimum once per day) all the LP tokens are sold for Sushi (on SushiSwap Exchange). The newly purchased Sushi is then divided up proportionally between all of the xSUSHI holders in the pool, meaning their xSUSHI is now worth more SUSHI. Because of the way the rewards are generated, the price of xSUSHI will increase with the value of SUSHI, and the value of one xSUSHI will always be greater than the value of one SUSHI. Fees generated into the Sushibar are vested for a period of time [see the Vesting FAQ], and to be eligible to earn them, your xSUSHI has to be staked here: https://exchange.sushi.com/#/stake.
If you want to have an idea of the current APY, you can check out here: https://analytics.sushi.com/bar.