This animation illustrates the "raisin cake" model of the expanding Universe. As the cake "bakes", it naturally expands in all directions, like a regular cake. The raisins, which are fixed in the cake, move away from each other as the cake expands. The raisins are analogous to galaxies, which for the most part move away from each other, because the space between them is expanding. This is what is meant by the expression "expanding Universe".
Raisins in the raisin cake migrate away each other with apparent "recessional" velocities that are a function of their original positions relative to each other before baking: the greater the original distance between any two raisins, the greater the "recessional" velocity between the two raisins.
This "distance vs recessional velocity" relationship is called Hubble's Law, and it looks like this:
v = H0 x d, where v is the current recessional velocity, H0 is Hubble's "constant", and d is the current separation distance between galaxies.
Note that Hubble's constant is not constant for all time, just more or less constant for this period of time in the Universe.
For more information on Hubble's Constant, go here.