Timing relates to where the cylinder is when the hammer falls. As a revolvers cylinder turns it brings the individual hole up so that the cartridge is lined up with the barrel and the hammer. If timing is off the cylinder may either not rotate all the way to the barrel or may rotate Past the barrel. When that happens it is said to be out of time. Usually it is caused by wear to one of the parts, either the piece that comes up from the frame to lock the cylinder in place (Colt calls that the bolt while S&W calls it the lock IIRC), the spring that pushes that up, the hand (part in the frame behind the cylinder that tirns the cylinder) or one of the various links or levers inside the frame. Colt has more parts and is a more complicated setup up than Smith (not to mention turning catawumps) and so are slightly more likely to get out of time. On the other hand, the more complicated Colt set up can be fine tuned better than the Smith.