The idea is that the volume is finite, yet unbounded. There is no way to picture this. But, we can picture a finite two dimensional object, that is unbounded.
If the area of the earth is finite, then what happens when you get to the edge?
You hold on for dear life.
Seriously though... stepping beyond Earth, you find yourself still within our solar system. When you reach the edge of the solar system, you find yourself still floating within the Milky Way Galaxy. Pushing beyond the bounds of our galaxy we find ourselves still contained within the Universe, but we continue to push ourselves further and further. Is that it? Does it just end there? Or is the Universe part of a more grand system? Does it keep going or would you eventually break on through... say, to the other side? (apologies to Jim Morrison)
The relative order of things would suggest that the Universe is limited such as Earth, the solar system, and the galaxy but that we only have yet to reach the limits of the "universe". And if we did ever reach it and passed beyond, what would you call it? Heaven? ;-) Or is it just a tiny organism contained within a petri dish in some heavenly laboratory?
And my apologies to the more studied in this field for my relative coarse awareness, I appreciate your knowledge and I really am curious. ;-)