Optical Image Stabilization
A technology for cameras that physically moves the camera lens or sensor to compensate for unintentional camera movement.
Any time a camera takes a photo, the image is captured over a period of time. That period of time is extremely short when working with ample light. But in low light, the period of time is usually longer, often long enough that even the smallest hand movement can blur the photo. So while OIS can be used for all photos, it's most useful in low light.
OIS uses a tiny gyroscope to detect those small hand movements and direct small actuators (motors) to move the lens in real time, in the opposite direction, to compensate. This keeps the image steady for the camera sensor even though the camera has moved.
OIS can only compensate for small movements.
The main alternative technology is EIS (electronic image stabilization), which does not use any physical moving parts. OIS is generally superior.