A layer 2 is a network built on top of a base blockchain, such as Ethereum, that processes transactions faster and cheaper while inheriting the base layer's security.
Base layers like Ethereum can become congested and expensive. Layer 2 networks move most activity off the main chain while settling back to it for security.
Rollups are the most popular, bundling many transactions into one. Other types include sidechains and state channels.
Arbitrum, Optimism and Base are well-known Ethereum layer 2 networks, each offering much lower fees than the mainnet.
Most wallets let you switch networks, and you can bridge funds from the main chain to a layer 2 to transact more cheaply.
Layer 2 networks make blockchains practical by reducing cost and congestion while keeping security.