What Is the Concept of ‘Stale Shares’ and How Do They Affect a Miner’s Profitability?

A stale share is a valid proof-of-work submitted by a miner that arrives at the pool server after the pool has already found and submitted a valid block to the network. Since the block has already been solved, the work represented by the stale share is wasted and does not contribute to the reward.

Stale shares reduce a miner's effective hash rate and, consequently, their profitability. High latency is a primary cause.