You move your mouse until you make the necessary entropy. You can use your keyboard as well with your mouse movements. After that, your private key is generated.
However, I read somewhere that humans are not good sources of entropy.... so maybe that is not a good idea. We problably keep moving the mouse in some crazy pattern.
I have seen that long ago, even Openbsd does it at first (ssh init?) boot (if you don't move the mouse it simply takes longer). Also noticeably Keepass and other password managers, before generating a random password.
I don't think they use only the mouse, but a combination of sources, just to make random more random
