So what happens is that when the cows have eaten the grass down to a certain level in a paddock (around 4 inches or so) they are moved to a new paddock and kept off of the paddock they were just on. This gives each paddock a chance to "rest".
Now this explanation leaves out a good deal of details and factors that have to be considered in rotational grazing. So if you plan to rotationally graze you will need to read up...read a lot. A good place to begin is
MANAGEMENT INTENSIVE GRAZING - By Jim Gerrish is a good book.
Salad Bar Beef by Joel Salatin is another good book, especially for a beginner.