The window concept in Jujitsu is a mental concept and idea that helps provide the symbolism or metaphor for how to start as a true beginner and advance with the right mindset mentally to help transition to a professional fighter.
The story starts with a number of open choices and then narrows over time. As a white belt, imagine being in a room with dozens of windows, and most of them will let you into the next room. In the next room some windows are locked or blocked in – so you have to find the right one to move forward.
When there is an open window, you move and adjust to take advantage. The same applies during the fight. When one move succeeds, quickly move to the next before that window closes once again. That is the successful use of the “window concept” in order to advance in fights and in belts.