The head nod is a familiar gesture that happens naturally to show agreement. The nod means that the listener is going along with what is being said but it can also be used as a tool to actively stimulate conversations. A slow nod indicates general agreement and that interest is present, whereas a quick head nod shows impatience and a desire to interject.
There are a variety of singular cues that also serve to indicate attention. Concentration for example, is obvious when the face is scrunched together, the eyes narrowed as if to focus on a prey, with the eyebrows tightened inward.