Overview
Selfish mining is a game-theoretic attack on Proof of Work blockchains. Instead of broadcasting a newly found block immediately, a 'selfish' miner keeps it secret and starts working on the next block. This gives them a head start on the rest of the network.
The Goal
By strategically releasing their private blocks just as the rest of the network finds a block, the selfish miner can cause other miners' work to be 'orphaned' (wasted). Over time, this allows the selfish miner to earn a higher percentage of rewards than their actual hash rate would suggest.
Impact
If successful, selfish mining can lead to increased centralization, as smaller miners are forced to join the selfish pool to remain profitable.