Blockchain networks are divided into layers based on their functionality. Base networks allow other layers to be built on top but service the underlying security requirements.
Some examples of Layer 1 networks include Bitcoin, Ethereum, Litecoin and such. The common issue with these networks is that of scalability, since their focus on security and decentralization compromises network speed and transactions per second.
To address this concern, developers build Layer 2 protocols on top of Layer 1 blockchain, to offload some of the network usage.