This is a good explanation from canon of their system:
Learn about image stabilisation in lenses and cameras. Find out about how image stabilisation keeps the image static despite the movement of the camera.
www.canon.co.uk
“Although an IS lens gives more opportunities for handheld shots, there will still be times when the support of a tripod is needed − with exposure times of several seconds, for example, or when you're working with heavy lenses.
With some of the earlier lenses, you needed to switch the IS off when using a tripod as the lack of movement confused the system and the image started to jump around the viewfinder. However, even when using a tripod, there can be some camera movement in high wind or with super telephoto lenses, which means the IS system can be invaluable. More recent IS lenses are able to detect the use of a tripod and automatically disable the IS, if necessary.
You should also leave the Image Stabilisation on when using a monopod, as it is unlikely you will be able to keep this type of support perfectly still”.