Camera shake occurs when hand-holding your camera. No matter how steady you think you are, you can never stand perfectly still, and this slight movement shows up in your photos as a blurriness or lack of sharpness.
You can avoid camera shake by using a faster shutter speed. It’s more noticeable when using lenses with a long focal length, so the longer the lens, the more you’ll need to increase your shutter speed to avoid camera shake.
As a rule of thumb, you should use a minimum shutter speed of 1/focal length. So for a 200mm lens, use a shutter speed of at least 1/200th. When calculating this, use your lens’s effective focal length, which is found by multiplying the focal length by your camera’s crop factor.