An underbase is a white layer of ink that the design sits on top of. This is very similar to laying down a primer or a base coat before painting a wall. It is used for dark garments to make your design show up, makes colors pop, and keeps inks from bleeding into the garment.
You do not need to provide this underbase layer yourself! This will be handled automatically upon printing 🙂
For a full guide to optimizing designs that require an underbase, check out the guide 'Designing for DTG: Prep School'.