The Springfield XD OSP is an optics-ready variant of the XD platform with a factory cut on the slide. The cut accepts standard plates for a range of red dot footprints.
A slide-mounted red dot lets you aim with both eyes open and pick up the aiming point faster than iron sights, then hold it through recoil — which is why most competitors move to one. The plate is the part that makes it work: it bridges the Springfield XD's mounting interface to the footprint of your chosen optic and keeps it sitting low and rigid over the bore.
Two dimensions have to line up — the cut or rail on the Springfield XD and the footprint of the optic you intend to run. A few plates include an integrated rear sight to preserve a co-witness backup. Check both against your setup on the individual product page before ordering.