SushiSwap pools (exchange.sushiswapclassic.org/#/pool) allow liquidity providers to add their tokens to liquidity pools. When they do so they will receive SLP tokens (Sushiswap Liquidity Provider tokens) for example if a user deposited $SUSHI and $ETH into a pool they would receive SUSHI-ETH SLP tokens. These tokens represent a proportional share of the pooled assets, allowing a user to reclaim their funds at any point.
As an LP its important to understand that you may be exposed to "Impermanent Loss" when providing liquidity.
However, as a liquidity provider you will also receive trade fee reward. 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 This means each LP token would be worth 1.00025 ETH and 1.00025 SUSHI now when it is withdrawn.
When you have your LP tokens you can also put them into yield farming on:
If you are farming on the farms, you will still get the 0.25% reward fee for being an LP when you next remove your liquidity. However, because the /farms and exchange.sushiswapclassic.org/#/pool are different smart contracts its important to know that the pool page cannot see the tokens you have in farms. When you are on the pool page it only scans your wallet for SLP tokens to tell you if you are providing liquidity. Therefore it may say "No Liquidity" even though you ARE providing liquidity. It's just that it cannot see which tokens you've deposited into the farms.