The SushBar located at sushiswapclassic.org/staking allows you to stake your Sushi and receive xSushi in return and then you stake it in the xSushi pool.
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 for the relative pool.
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 the xSushi holders in the pool, meaning their xSushi is now worth more Sushi.
Currently you will not see the gained amount until you have withdrawn.
It started as 1 Sushi = 1 xSushi, but just like LP tokens the price of xSushi changes over time depend on how many Sushi rewards are in the pool.