In app development, a task is a single, discrete unit of work that needs to be completed to move the project forward. It's the most granular level of work item in a project plan. Tasks are actionable, assignable, and should be small enough to be completed within a few hours to a few days.
Think of building a house: "Build the north wall" is a feature; "Hammer nail A into stud B" is too granular. A good task is something like "Frame the north wall" or "Install electrical wiring in the living room." In app terms, "Implement user login" is a feature, while "Create the Login Screen UI" or "Integrate the Login API" are tasks.