An L1 (Layer 1) blockchain is any blockchain that safeguards their own security in the form of proof-of-work, proof-of-stake, or any other consensus mechanism. Examples of these could be Ethereum, Avalanche, Solana, etc.
An L2 (Layer 2) blockchain is any blockchain that is built upon a layer 1 blockchain, providing better scalability and likely inheriting the security of the original chain. Examples of these could be Polygon Hermez, Optimism, Arbitrum One, ZkSync, Loopring, etc.