Rotoscoping is a trick used in animation. Using frames from live footage, animators would trace over the bodies to make their character's body movements look realistic. Otherwise, Ariel using her dinglehopper here wouldn't have looked right.
The technique wasn't invented by Disney (that credit goes to Max Fleischer, creator of classic characters like Betty Boop, Popeye and Koko the Clown). Still, the Disney company has made use of it in some of their greatest movies.