‘Tracking’ in this case means recording audio through a microphone. If someone is singing, while also listening to the rest of the song through headphones, it is possible that the music coming through the cans is picked up by the microphone, which is usually considered undesirable.
The DT 7700s are a “closed” design, with plastic all around the outside to help the sound stay “inside” the headphones and away from the mic. The DT 880s are “open”, so a lot more sound gets out. That’s fine if there is no microphone around, like when mixing, maybe not so much when tracking.
For the rest of it, sounds like you are on the right path, a little “listening exercise” every day! Others can give you far better advice than I, but maybe one exercise to try (if your version of Cubase supports it) is to listen to a commercial track with the pan controls full wide, then shrink the panning field so the blue area becomes smaller and smaller, until ultimately it becomes mono. At each stage of narrowing of the stereo field, it should sound different.