An anti-Sybil mechanism is like having a neighborhood watch that requires each member to prove they actually live on the block. In Bitcoin, proof-of-work is one such measure, making it costly to create multiple mining identities and sway consensus. Other systems might rely on identity verification, staking, or random selection.

Without anti-Sybil measures, an attacker could flood the network with fake nodes to influence block validation or disrupt communication. These defenses help ensure the network remains decentralized and trustworthy, although they often introduce trade-offs in cost, resource usage, or degree of centralization.