Clamped keys are keys that tell the foot control animation to stop at a certain point and not move or slip as the other parts of the body move. I think your foot keys are made up of spline keys, your animation on the feet look like he was floating or skiding across the ground.

To hopefully fix the problem go to your maya window go to window-Animation Editors - Graph Editor. Once the graph editor is up select only the foot controls for both the left and right feet and select thoughs keys in the right window. In the graph editor window go Tangents-and Select Clamped. Now if you keyed your feet correctly when you play back your animation the feet should stick to the ground as one foot makes his step then the other will follow only after the first foot has finished.

Hope its not too confusing, would be so much easier to understand if I could show you.

PS.. Try adding some rotation and up and down even though you think it might look funny it should actually make it more realistic.

Kurt