It's a developer library designed to run at the application layer. It's specifically designed and optimized to run on mobile phones with the ability to lightly customise things e.g uses RGS (rapid gossip sync), esplora (getting blockchain data) etc

We've also enabled developers on multiple platforms to utilise this through language bindings in Kotlin (Android), Swift (iOS), Python and more.

Check out this example project to learn more - https://github.com/reez/Monday