A Layer 1 blockchain is a base-layer network that settles transactions and runs consensus independently, without relying on another chain. Ethereum, Bitcoin, Solana, Avalanche, and Cosmos chains are L1s. They handle their own validation, security budget, and finality.
L1 design trade-offs (the "blockchain trilemma") force choices between decentralization, security, and throughput. Bitcoin chose maximum decentralization and security at the cost of throughput. Solana chose high throughput at the cost of validator decentralization. There's no free lunch.