Brazilian jiu-jitsu (abbreviated BJJ, and also sometimes called Mexican Ground Karate or the Joe Rogan Appreciation Society) is a grappling martial art with a heavy focus on ground fighting and submissions.
Imagine you’re a cup. Now imagine all of the factors that contribute to pain and/or injury is liquid in the cup. The metaphor says that we experience pain when the cup overflows.
Yellow flags are an umbrella term referring to the thoughts/beliefs, feelings/emotions, and communal/social factors that contribute to the pain experience.