ORIF means Open Reduction-Internal Fixation. It refers to the treatment of a fractured bone, often of the femur - the thigh bone which is the longest bone in the body. ORIF can be (and is) used to refer to fractures in other bones in the body.
An ORIF refers to the treatment selected by the surgeon.
The operation will consist of opening the skin, for example on the thigh, to get to the fracture (Open Reduction) as opposed to putting the leg or arm (i.e., bone) in a cast and never cutting in the skin (Closed Reduction).
Internal Fixation means that once the break is exposed, the bone will be stabilized to allow it to heal with either screws and plates or a rod inserted into the center of the bone. External Fixation would be either applying a cast to the arm or leg (i.e., the bone) or putting rods through the bone with plates and screws that sit on the outside of the skin.