RMR stands for "Ruggedized Miniature Reflex" sight. It is a type of red dot sight commonly used on handguns and other firearms. RMR sights are small, lightweight, and durable, making them popular choices for enhancing target acquisition and accuracy.
An RMR cover plate, would refer to a small metal plate that is used to cover the area on a firearm where an RMR sight could be mounted. Some handguns come with "optics-ready" slides, which have a milled area, or a set of mounting holes specifically designed to attach an RMR sight directly to the slide.