The object in motion, indicated by the heavy circle, is undergoing motion with no acceleration. This means that the velocity of the object is constant. In more than one dimension,both the speed and the direction of the velocity (and any other vector) must be specifed. The short line coming off of the top of the object represents the velocity of the object, which is constant in size and direction. Any change in the velocity of the object (size or direction) would correspond to a non-zero acceleration.