What is a stale block?
A valid block that lost the race: another block at the same height was built on instead, so the stale one is not in the chain.
Two miners sometimes find a block at almost the same moment. Nodes keep the one that becomes part of the chain with the most work; the other goes stale, and its miner loses the reward.
A stale block is not an attack, just bad luck, and the race resolves within a block or two. Payments in the loser return to the mempoolMempoolThe waiting room for Bitcoin transactions that have been sent but aren't in a block yet. if the winner didn't include them. Older writing calls these orphan blocks.
References
- Bitcoin Developer Guide, “Block Chain: Block Height And Forking”.