It means that everything you need to arrive at a consensus on the validity of a transaction is on the blockchain, and thus enforced by the decentralized network

It means that everything you need to arrive at a consensus on the validity of a transaction is on the blockchain, and thus enforced by the decentralized network of nodes that validate the blockchain. So contrary to what the grandparent comment claims, these layer 2 (i.e. scalability) solutions make no compromises on decentralization, as they would if they had a consensus-critical component that was managed by a centralized off-chain party.