IBIS is In Body Image Stabilisation.
It is a stabilisation system in the camera body, opposed to stabilisation that you will find in a lens. Some camera systems allow both the in body and the in lens stabilisation systems to work together to maximise the effect.
The point with stabilisation is to mitigate camera shake. If you shoot with a slow shutter opening time, it is possible to move the camera a good deal while the shutter is open. This will be visible in the image; it may look blurred or not sharp.
The classic way to mitigate this is via the use of a tripod to stabilise the camera. An alternative is to follow the reciprocal rule, that says that the shutter speed should be equal to or faster than the reciprocal focal length. So if you are shooting at 100mm, you should not shoot slower than 1/100th of a second.
IBIS in the camera body allows you to shoot handheld at slower shutter speeds than what the reciprocal rule suggests.
Related reading
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What is banding in photography?
