Other forms: obis
An obi is a fabric belt that's tied around the waist of a Japanese kimono. The most formal obis are very wide and stiff, and are made from silk brocade.
While an obi was originally a useful piece of cord or rope that secured a kimono around the body, modern obi are primarily ornamental. These wide sashes have become too inflexible to actually hold the kimono shut — there are smaller ties underneath that do that. If you participate in Japanese martial arts, you'll also wear an obi as part of your uniform.