A layer-1 (L1) is a base blockchain that provides the underlying infrastructure for the network.
The Long Explanation
Layer-1 are blockchains in their purest forms. This is the protocol that provides the main blockchain architecture. This means it processes and finalizes transactions directly on its own blockchain, and it has its own native token for settling transaction fees.
Good examples of layer-1 blockchains are blockchain networks like Ethereum, Bitcoin, or BNB Chain. Unfortunately, as the understanding of blockchains improves, layer-1 blockchains may find themselves stuck with certain inefficient processes.
It is this need for scalability that has seen the development of other blockchains on top of existing blockchain networks to improve upon the underlying infrastructure like decreasing transaction fees and improving efficiency.