What is meant by Discovery stage?

The most important idea of the Discovery Stage is estimating product development. Initially, it is done via Project Vision & Scope documents, that are being written during the Discovery Stage: the team prepares product requirements and project planning documentation which is used as a basis of the development.  This documentation includes the most essential for the project launch:

1. Business and other High-level requirements;
2. User requirements and Functional requirements;
3. Wireframes and Designs;
4. Project plan.