A layer-1 blockchain is a set of solutions that improve the base protocol itself
A layer-1 blockchain is a set of solutions that improve the base protocol itself to make the overall system a lot more scalable. There are two most common layer-1 solutions, and these are the consensus protocol changes as well as sharding.
When it comes to consensus protocol changes, projects like Ethereum are moving from older, clunky consensus protocols such as proof-of-work (PoW) to much faster and less energy-wasteful protocols such as proof-of-stake (PoS).
Sharding is one of the most popular layer-1 scalability methods out there as well. Instead of making a network sequentially work on each transaction, sharding breaks these transaction sets into small data sets which are known as "shards," and these can then be processed by the network in parallel.
One of the pros when it comes to layer-1 solutions is that there is no need to add anything on top of the existing infrastructure.