Layer 2 refers to the off-chain network, system or technology based on the underlying blockchain (note: also commonly referred to as the "Layer 1 network"), with the purpose of expanding the underlying blockchain network. The Layer 2 network can improve the throughput and other performance of any underlying blockchain.
The core value of a Layer 2 network, system, or technology is the ability to leverage the security of the underlying blockchain. Its transaction data must be verified and confirmed in some form by the underlying blockchain network. According to this standard, sidechains do not belong to Layer 2 because sidechains usually deploy their own consensus mechanisms and verification nodes, and therefore have security mechanisms independent of the underlying blockchain.
Some blockchains sacrifice scalability in order to ensure decentralization and security. Such blockchains can use Layer 2 to increase transaction throughput and reduce transaction costs. Layer 2 is one of the solutions to the scalability problem, which can quickly execute transactions and achieve scalability without sacrificing decentralization or security.