Imagine a creature, about the size of a football, which has long green fur, and several purple tentacles with a glowing red eye at the end of each.
You have never seen such a creature; you are never likely to see such a creature. But I bet you were able to imagine such a creature, and perhaps your mind also filled in additional details than the ones I described. Perhaps you added a mouth, teeth, perhaps the creature moved, the wind ruffled its fur. Maybe it had feet. Maybe it made a sound. Maybe you found that a whole environment sprung into existence around it.
Now try to imagine a sphere, a snooker ball say, which is spinning in one direction, and also, simultaneously, spinning in the opposite direction.
Again you have never seen this, but this time, you could not imagine it.
The simple difference is that the creature, however unlikely you are to ever encounter it in your day-to-day life, is possible. It is entirely feasible that this very creature exists somewhere in the universe. The ball spinning in opposing directions is impossible, by the very definition of “spin”.
Now, something amazing comes out of this seemingly futile exercise. Your mind is capable of imagining only what is possible. If something is possible, you can imagine it, just by someone describing it to you, even if it bears little or no resemblance to anything you have seen before — provided it can be described in terms that you understand. But if it is impossible, you cannot, no matter how well it is described.