Lagom provides tools and APIs that guide developers to build a system out of microservices. The key to success is good architecture, rooted in a solid understanding of microservices concepts and best practices.
The philosophy behind Lagom is presented in Reactive Microservices Architecture: Design Principles for Distributed Systems, a report by Jonas Bonér, available free. We strongly recommend reading it before learning Lagom. Much of the following documentation builds upon and refers back to this report.