The phrase "shelf unit" refers to a piece of furniture that contains multiple shelves, typically used for organizing and storing items. Full definition
Open metal shelving units in this space will come in handy for easy access to and storage of small and medium sized boxes of items and materials, such as plumbing and lighting fixtures. (trippalukastyle.com)
We created more storage my adding two open shelving units on the wall. (beneathmyheart.net)