Double-Action Only denotes a Revolver that cannot be manually cocked. As with other Double Action Revolvers, pulling the trigger cocks the gun and advances the cylinder, but in this case you can't choose to do so without pulling the trigger. The hammer may even be entirely hidden and inaccessible without disassembling the gun. Most of these are intended for Concealed Carry, and the is designed to reduce the chance of the hammer catching on clothing. This is also helpful during the Quick Draw. As with any Double Action Revolver, expect a slightly slower rate of fire (than a Single Action Revolver), but improved accuracy.
The main problem with double-action only weapons is that a double action shot requires a much heavier trigger pull that a simple action shot, and this generally makes it less accurate - hence the common preference for dual action weapons when they are available.
Unfortunately, being unable to cock the hammer takes away some of the Cool Factor of a Revolver.