Blockchain nodes are devices that are part of a blockchain network, which can either serve as full nodes, which store the entire copy of the blockchain, or light nodes that only store part of the blockchain. Full nodes validate transactions and blocks against the rules of the protocol, ensuring that all transactions are legitimate before they are added to the blockchain. They also propagate new transactions and blocks to other nodes in the network. Light nodes, or SPV (Simplified Payment Verification) nodes, do not download the entire blockchain and rely on full nodes for verification, making them less resource-intensive and faster to operate. Nodes play a critical role in the decentralization and security of blockchain networks, enabling trustless interactions without a central authority.