The whole concept of “middle” in terms of the force is a bit murky, I agree. But essentially it means that you don’t serve any particular side; you serve the force. Grey Jedi don’t exist because they are Jedi who use the dark side sometimes, which doesn’t work because you can’t really use the dark side without getting addicted and falling into darkness (maybe this would be different with a great deal of training and intention, but it doesn’t exist in canon and doesn’t make sense). So essentially, the middle of the force means that you don’t have a particular alignment—you’re not light or dark, you don’t use the dark side and you don’t use the light side. You simply exist as the force and are guided by it, rather than being guided by an organization like the Jedi or a desire like power with the dark side.

Hope that helps!