Mantra is an application architecture specification and set of libraries for building Meteor apps. It is based on Flux principles and includes core components like React UI components, action handlers for business logic, state managers for local and remote data, and container components to connect UI, actions and states. While Mantra provides a good architecture, the author believes it could be improved by being tailored more specifically for Meteor, enhancing unit testing support, and developing a code generation tool or opinionated framework to implement the pattern.