What are orphan blocks and why do they hurt security?

An orphan block is a valid block that the network discards because a competing block was accepted first. When a blockchain's block creation rate is too fast relative to how long it takes blocks to travel across the network, many miners end up building on different versions of the chain at the same time — and only one version wins. Every discarded block represents real computational work that earned nothing and contributed nothing to the chain. Orphan blocks matter because that wasted work represents mining power that did not go toward securing the network, which weakens the overall security guarantees that proof-of-work is designed to provide.