What Is BJJ Open Mat? 

Open means that the gym is open for use, but there is not a scheduled class. Open mats are a time when you are free to practice what you want at the gym. As there is no class, you can drill techniques, spar, or work on your fitness. 

Before attending an open mat you should make sure to follow all of the protocols of your gym. Just because there is no instruction, doesn't mean that anything goes.