To illustrate, let's say a single BUSD-BNB LP token has 1 BNB (worth $600) and 600 BUSD as the underlying asset, therefore it is worth $1,200 per LP ($600 + $600). When someone swaps on exchanges, they pay a fee, and those fees are distributed to liquidity providers (represented by LP tokens). Say someone paid $600 in fees and your single LP token represents 10% of the pool, you'd immediately get $60 (10% * $100; split in half between BNB and BUSD) added to your underlying asset. Now your LP is worth $1,260, rather than $1,200; 1.05 BNB and 630 BUSD.