Basics of Agile
How Product, EPIC, Theme, and Story are Releated with each other

Product: A product represents an item or a feature that is to be developed and released to the market.
Theme: Area of focus with an associated business value. A theme is related to one or more of the company’s goals.
Epic: High-level definition of a requirement that provides value to the business, such as a new feature or a significant enhancement. Epics are broken down into Agile stories and can be worked on by a single or multiple teams.
Story: Short, manageable pieces of work that are related to an epic. Stories capture the who, what, and why of a requirement in a simple and concise way.
Backlog: List of work that must be implemented, as part of achieving specific outcomes. Backlog contains work related to new features, enhancements to existing features, and other activities of product development.