An ambulatory surgery is the kind of procedure where the patient walks in and walks out. Ambulatory means able to walk, or related to walking.
To remember ambulatory, think of ambulance, which essentially means a walking hospital. (Its meaning derives from the time when it was pulled by horses, which would be walking.) You can also think of the old fashioned word for baby carriage, perambulator (it's been shorted to "pram") which means a carriage you push by walking.