You've learned how to create a subgraph from SushiSwap exchange. Your subgraph is tracking the state of the market for every existing Sushiswap pair, e.g. amount of both tokens in WETH-DAI pool. Additionally, position of every user, or more precisely liquidity provider, is tracked as well. When user provides his tokens into the pool, he gets back Sushi LP (SLP) tokens. You can find out how many SLP tokens user held over time, how much trading fees he earned and what was the ROI on that investment at any point of time.