its vibrato. listen to singers. you know how their voice doesnt just stay flat when they hold a note? how it kind of shakes? thats vibrato. its an expressive effect. the easiest way to do it on a saxophone is to make your jaw move up and down a little bit. youll notice the pitch goes up and down with your jaw. just do this until you have good control over it. try setting a metronome and going down for four beats, then up for four beats, than go to two beats, then one beat, than eight notes, then triplets, then sixteenth notes, etc., until you can do anything you want with it. then listen to a bunch of musicians' vibratos and come up with something that you think sounds good