Laboratory tube racks are used to store (micro)centrifuge tubes on your benchtop. This keeps the tubes nice and handy for quick loading/unloading, storage, or just for appearing to be organized even though you really aren't.
There's no shortage of tube rack variety. There are connectable 4-way racks the colors of a children's toy, which is endearingly unpretentious. There are lots of plain white polypropylene ones if you're both boring and economical (like me). There are even wooden and wire tube racks.