Ethereum and Bitcoin do have scaling problems. This is a fundamental problem of blockchain. So more thinking about adding another layer on permissionless blockchain, that's what we call layer 2 solutions.
Layer 2 solutions contain several type of solutions: some to scale payment, some are used for scaling smart contract, and some to do computation off chain. But one feature they are the same: move most of the work off chain and use permissionless blockchain as anchors to ensure security.
Nowadays, there are several branches of off chain scaling solutions: State Channels, Sidechain (Plasma), Off chain computation (Truebit, TEE). In this feature, we use “Layer 2” to refer off chain scaling solutions.