A rashguard is a compression-fit shirt made from stretchy, moisture-wicking fabric — typically a nylon-spandex blend. In no-gi BJJ it is the standard top layer, worn under a gi jacket or alone for no-gi training. Its main jobs are reducing mat burn, preventing skin abrasions, and keeping sweat off your partner.
Most academies require a rashguard for no-gi classes. Some also require compression shorts or spats (full-length tights). The tight fit means there is nothing to grip, which is intentional in no-gi — without gi cloth, you develop clinch, underhook, and body control rather than fabric-based grips.
Rashguards often display belt rank (matching colour stripes or coloured body panels) so partners can gauge each other's level in no-gi classes.