The phrase "to add on" means to increase or enhance something that already exists. It can refer to adding extra features, components, or services to an existing product, building, system, or process in order to improve its functionality, value, or performance. The term is often used in the context of construction where additional rooms, floors, or extensions are added onto a pre-existing structure. In general usage, "to add on" can also refer to expanding or extending something over time, such as adding more responsibilities, tasks, or activities to an existing job, project, or routine.