Do you mean the crank has "play" in it-- that is can "wobble" back and forth in the frame?
If this is the case you want to tighten the left side cone. Take off the large hex lock nut. Then remove any loose spacers. You should now see a round disc-shaped piece of metal in front of you with some slots in it. Take a flat bladed screw driver and tighten it (turn it COUNTER clockwise b/c you're on the left side). Tighten it until you have no play left, but the cranks still spin freely.