The term for someone pointing at an object to attract attention to the object
In painting, this figure is called a festaiuolo:

[Artists of late medieval and Renaissance Europe] relied on festiauolo - figures who attract our attention by glance or gesture, point us in the appropriate direction, and indicate how we should respond or behave. The concept of the festaiuolo was taken by artists from the Italian theatrical and celebratory conventions. On stage, he or she was usually a secondary character who served as an intermediary between the audience and the main action, explaining, commenting, and modeling the appropriate response. The meaning of the word itself, literally &quot;feast-maker&quot;, suggests the role of master or mistress of ceremonies, who would perform introductions and direct guests, and so the festaiuolo functioned in a work of art.
Scholarship and Christian faith: Enlarging the Conversation; Douglas Jacobsen, Rhonda Hustedt Jacobsen; 2004. p. 144

This pointing can be to something or someone inside the painting, as well as outside of it, in which case it usually is a concept, like God or Fate (here is a related thread on History.SE).