Selfish Mining refers to a situation wherein a cryptocurrency miner mines a new block without having to disseminate the information to other miners.
Should the miner be able to find a second block faster compared to the other miners, then they were given the chance to produce the longest public chain. This would in turn, invalidate the other blocks that have been discovered within the timeframe that the attack took place.
The Selfish Mining strategy is done so that miners to collude to gather further revenue. Selfish Mining can lead into the centralization of Bitcoin mining operations.