Usually caused by shooting although some endshake is present from the factory in many guns. Push back and forth on the cylinder with the yoke/cylinder assembly closed into the frame. That's endshake. Whether it's too much depends on whether the gun will fire reliably or not. In 40 years of shooting I can honestly say I've never run into a revolver with excessive endshake to the point reliability was in question although I do have a few washers on hand.....just in case! There are some books out there which will give you specific information on what's acceptable and what to do about it.