What is the Layer 2 ?

A layer 2 refers to any off-chain network, system, or technology built on top of a blockchain (commonly known as a layer-1 network) that helps extend the capabilities of the underlying base layer network. Layer-2 networks can support any blockchain to introduce enhancements such as higher transaction throughputs.
One core requirement for a network, system, or technology to be considered a layer 2 is that it inherits the security of the blockchain it is built on top of. Transaction data must, in some shape or form, be verified and confirmed by the underlying blockchain network rather than a separate set of nodes. For example, sidechains are often not considered layer 2s because they usually deploy their own consensus mechanisms and validators, leading to a different set of security guarantees than that of the base layer chain.
For blockchains that sacrifice scalability to achieve higher decentralization and security, layer 2s enable greater transaction throughput, which can lead to lower fees. Layer 2s can be seen as one solution to the problem of scalability, enabling fast and scalable execution without compromising on decentralization or security.