Discovery Phase is the stage of active research at the very beginning of a web application development process. Usually UX Designer, Tech Architect, and/or Project Manager take part in the discovery phase research. They try to establish business goals, see if this project matches the company resources and technical stack, understand the current stage of the project, its background and suggest alternative solutions if possible.
The essential purpose of the discovery phase is to reduce ambiguity by getting the direction and goal for the project development team. It also identifies possible roadblocks and milestones.
A Discovery Phase can be integrated into any management methodology: Agile, Kanban, Waterfall, etc.