What is weak subjectivity in proof-of-stake?

Weak subjectivity is the approach most proof-of-stake (PoS) chains use to guard against long-range attacks: nodes are required to trust a recent checkpoint provided by someone they consider reliable, rather than deriving all security from the chain's own history. In a fully objective system, any new participant can verify the entire blockchain from scratch using only the protocol rules. Weak subjectivity breaks that guarantee — a node that has been offline too long, or a newcomer syncing for the first time, must consult a trusted source to identify the real chain. This matters for beginners because it means PoS chains reintroduce a degree of social trust that proof-of-work is designed to eliminate entirely.