A razor strop is a tool used to sharpen a straight razor (also called open razor and cut-throat razor).
A razor strop is basically a wide strap of leather, about one foot (30 to 40 cm) long, with a D-ring, bail or loop on the upper end and a handle on the lower end. The strop is hung, e.g. from a hook on the wall or a door knob, and pulled tight with one hand while the razor blade, held in the other hand, is sharpened on it. The sharpening on a strop is done with the blade edge, not against it as you do in honing or grinding. No material is reved from the blade by sharpening it with a strop; the indentations are only re-aligned.
The type of leather used is usually cow hide. Sometimes a strop is made from two straps of leather. Sometimes it has leather on one side and canvas on the other, or is fully made from canvas.
 Use for corporal punishment
Razor strops used to be common household (bathroom) items in the days when most men shaved themselves with straight razors. Occasionally a razor strop was also used by its owner, the male head of household, for the administration of domestic corporal punishment. Such a punishment was called a stropping. Often the terms "stropping" and "strapping" can be used interchangably.
The metal part on the strop's upper end was certainly not ideal and in some cases where the strop was no longer fit for razor sharpening after years of use (having become too soft), but still usable for CP, that end might have been cut off resulting in just a wide strap on a handle.
Modern designers and producers of spanking implements catering to the BDSM and spankophile clientele sometimes call their implements strops when they are based on the razor strop design. There is no clear technical distinction between such "strops" and "straps". There is also significant overlap with a doubled-up belt, the slapper and the leather paddle.