Prehension in philosophy and by definition means an interaction of an organism with a certain entity or event. For an infant who is 5-month-old, the prehension that is most commonly observed is the grasping of objects voluntarily.
This happens as they begin to consciously use their fingers in order to take hold and grasps objects. As of this time, they have not learn the finger isolation so they tend to use all their fingers as a whole entity.