The pool returns graph shows the performance of the pool over the last month in USD terms. "Pool Returns" tracks the USD value of the pool's assets plus any trading fees earned. "HODL" tracks the USD value of the pool's assets as if they were sitting in your wallet.
"Pool Returns" will reflect the impact of impermanent loss as well. If "Pool Returns" is negative, that means the value of the pool's assets have declined in USD value over the last month.