Every time a user trades between $SUSHI and $ETH a 0.3% fee is taken on the trade. 0.25% of that trade goes back to the the LP pool.
If previously there were 100 SLP tokens representing 100 ETH and 100 SUSHI each token would be worth 1 ETH & 1 SRM.
If one user trade 10 ETH for 10 SUSHI, and another traded 10 SUSHI for 10 ETH, then there would now be 100.025 ETH and 100.025 SUSHI.
This means each LP token would be worth 1.00025 ETH and 1.00025 SUSHI now when it is withdrawn.