A project contains a collection of assets and the data needed for sharing and testing those assets. Users can edit a project and its assets in UEFN and in Creative Create mode where applicable.
A project can have multiple assets. It can also have multiple levels (or no levels), and meshes, textures, sounds, and other characteristics, depending on the needs and intent of the project the user is building.
You could also say that a project is a single source of truth for a game or content library.