#AI Servo Autofocus
AI Servo is the autofocus mode on Canon cameras that continuously tracks a subject. As long as you are half-pressing or fully pressing the shutter release, (or using the popular AF-ON technique) the camera's autofocus system will continuously track whatever subject is under your selected focus point.
This autofocus mode is useful for tracking moving subjects, and is also useful for focusing at very shallow depth (fast apertures) when shooting hand-held in very close-up situations. (Focusing on a bride's eyelashes using an f/1.4 aperture lens, for example)