Quite literally, the term, Second Layer Solution, pertains to a set of solutions built on top of a public blockchain.
The reason behind for the creation of this solution is imbue the relevant blockchain with greater scalability and efficiency, with consideration to microtransaction. Good examples of Second-Layer Solutions are Bitocin’s Lightning Network and Ethereum Plasma. The former is based on what are called state channels, which are attached channels able to perform blockchain operations and make reports to the primary chain.
The protocol efficiently solves issues in transaction speed and scaling challenges that are faced by even the most well-known of cryptocurrency networks, hence the aforementioned cryptocurrencies.