Layer 1 is simply the main blockchain architecture. Bitcoin and Ethereum are layer 1 architecture, while layer 2 is anything built on layer 1 to make it more efficient. With the evolution of blockchain technology and the DeFi boom, there became a need for blockchains to become scalable and more efficient, creating layer 1 and layer 2 solutions.
Layer 1 solutions are upgrades of the main architecture without an overlaying addition. Improvements to the main architecture can come through sharding and a new consensus mechanism. An example of this is the Ethereum blockchain switching from Proof-of-work to Proof-of-Stake. This switch is based on the main architecture itself and not through side chains to make the network efficient.
Layer 1 has several advantages because there is no need to add a new chain or related improvements to the network through layer 2 solutions that have been seen to bring about faster transactions without disrupting the underlying architecture.