The expansion of the universe means that select one:
a. as time goes by, galaxies move away from each other through empty space.
b. as time goes by, space itself expands, carrying the galaxies along with it.
c. each object in the universe expands its size.
d. the light from distant galaxies is doppler shifted as the galaxies move away from us.

Scientists beleive that the universe is constantly expanding since the big-bang, and that the galaxies are moving away from the epicenter of said big-bang. Now this could mean A or B is the answers, however A. states that they move through empty space, which is true but doesn't explain how the universe is expanding. B does however explain this as space expanding, which accounts for all the empty space being created by the galaxies moving apart.

Hope this helps.