It is the injection counterpart of the productivity index, in barrels per day per psi. A clean injector has a stable index. The index falls when the sand face plugs with solids, scale, oil carryover, or bacteria, and that fall is often the first sign, before the surface pressure hits the limit of the pump.
Comparing an injectivity index to a productivity index assumes the same fluid and a similar completion. Water is less viscous than oil, so a water injector can take a higher rate than an oil producer in the same rock for the same pressure difference. A falling index that is really a rising reservoir pressure, because the voidage is not balanced, is not plugging.