To motorize is to give something a motor so that it will move or work better (or faster). If you motorize your bicycle, it'll make riding up hills a lot easier.
Some wheelchairs are propelled by turning the wheels by hand. If you motorize a wheelchair, it helps someone with little arm strength get around. This verb is also used, especially in military contexts, to mean "equip with vehicles," so a commander might decide to motorize an urban unit during a conflict, setting it up with motorized tanks to patrol more safely.