A Layer 2 (or L2) is a network built on top of a main blockchain (the Layer 1, such as Ethereum) to process transactions faster and more cheaply, while relying on that blockchain's security.
The most common form is the rollup: it executes a large number of transactions off the main chain, then publishes a summary to it. Optimistic rollups (Arbitrum, Optimism, Base) treat transactions as valid unless challenged; zk-rollups (zkSync, Starknet, Linea) publish a cryptographic proof of validity.
L2s make applications usable that Layer 1 fees would make too expensive: DeFi, payments, games, NFTs.
Why it matters when hiring
This mostly matters for Web3 teams. A serious smart contract developer can explain the differences between rollup types (withdrawal delay, security assumptions, compatibility with the Ethereum virtual machine) and the risks of cross-chain bridges. Be wary of profiles who only name networks without understanding what they guarantee.