Product review (5 stars): Fixed my washer in about 5 minutes
Agitator dogs are made out of plastic.  Over time they wear down and no longer engage the ratchet mechanism inside the agitator.  When this happens the agitator spins freely in both directions, decreasing the cleaning ability of the washer.

These are very easy to replace.  They did not come with instructions, but there are many videos on youtube which will explain how to do this.  You will need a long nutdriver to perform this work.  I used a 1/4" ratchet with a 7" extension, IIRC.  The repair took approximately 5 minutes.