The motion usually consists of a side-to-side tilting of the head in arcs along the coronal plane.
[1] A form of nonverbal communication, it may mean yes, good, maybe, okay, or I understand, depending on the context.
The meaning of the head bobble depends on the context of the conversation or encounter.
It can serve as an alternative to thank you or as a polite introduction, or it can represent acknowledgement.
An unenthusiastic head bobble can be a polite way of declining something without saying no directly.