GHOST originally was a protocol modification, a chain selection rule, that makes use of blocks that are off the main chain to obtain a more secure and scalable system. It adopts orphan blocks – the valid blocks that did not make it to the main chain, as uncle blocks so that work from these blocks is included on the main chain.

This prevents the attacker to overtake the mainchain. It is more relevant and applicable in Blockchains where time taken to create new blocks is quite less. Eg: In Ethereum, block time is 10 to 20 seconds while in Bitcoin it is 10 minutes. Hence, in Ethereum the chances of orphan blocks or stale blocks are very high.

Instead of ignoring these blocks, Ghost protocol considers them while determining the longest chain to ensure network security and prevent centralization.