A speedloader holds six (five) cartridges in alignment at the same spacing as the chambers of your revolver cylinder. Just poke all six rounds into the chambers at the same time and, depending on the brand of speedloader, either twist a knob about 60 degrees, or just push hard to unlatch the cartridges and let them all fall into the chambers at the same time.
Speedloader is specific to the make and model gun with some overlap. No modifications to the gun required except that the left side grip must clear. They are practically essential for competition and for duty by the few LEOs still carrying revolvers. They are a little bulky for John Q Public to carry but they have the advantage of holding a reload in one package. Belt carriers are available and they ride pretty well in a side jacket pocket.