Location: Budapest, Hungary
It is hard to grasp of course, because it concerns with a different approach to physical time as humans now know it. Multiple theories exist regarding the nature of time and time perception, and the film concerns itself with the notion that for certain sentient beings, maybe time isn't a linear, "arrow-like" thing, but rather a canvas where you can see the whole thing instead of living it through a series of moments.
This is of course really hard to grasp, because our minds were not evolved to think of time as a canvas. It's the same problem as with quantum theory - physicist can work with it, explore it, but no one can really GRASP it because it is perfectly contradictory to everything we've ever known about the world. Still, it is true.
So it is perfectly reasonable that you did not get that part, or that you cannot fully grasp it. Because no one can. The rules of the world in the tesseract are in contradiction with common sense, so asking things such as "so how could they communicate BEFORE or AFTER this and this" makes no sense. There is no before or after.
Several people ask: what happened BEFORE the Big Bang? Seems like a legit question, right? But Stephen Hawking says that the question itself does not make sense - time itself began with the Big Bang, so there is no "before" it. No use asking these questions.