The top part of the agitator turns by hand but not during wash or rinse cycle. The bottom part still works so the clothes dont rotate, the top ones stay on top and the bottom on the bottom.
You need to replace cams in the top part of the agitator. They are called "dogs".
It is an easy and very inexpensive repair, will take about an hour or so and it helps to have a helper.
Here is a site that will show you how it is done.