Replacement flat wire recoil spring. Some fitting may be required.
Current production factory Glock springs are 16 pounds. Early, non-captive versions of the 17 and 17L have recoil springs of 19 pounds. Recoil springs fit over a pistol's guide rod and control how the pistol's slide functions.
In general, heavy loads need heavy springs and lighter loads need lighter springs. There is no hard and fast rule as to how much "Spring" a pistol needs. Too light a spring and the pistol opens and closes early and hard, increasing the felt reoil and battering the frame. Too heavy a spring and the slide is hard to open and may not cycle completely, jamming the pistol.
Shooters who are using +P ammunition or self defense ammunition will likely need heavier recoil springs. Shooters who are using target ammunition may be able to use lighter recoil springs.
A non-captive guide rod spring is highly suggested. It keeps you from having to disassemble the factory Glock captive guide rod. Glock 17, 22, and 24 Guide Rod #114-642. Glock 20 and 21 Guide Rod #119-858. Glock 19 and 23 Guide Rod #719-532.