A vertex is a point in space that forms one of the three points of a polygon. It can be represented with an XYZ vector, such as
(0 0 0).
Vertices (not "vertexes"!) are by themselves invisible. It is the polygon they come together to form that is rendered. However, they do contain a 'normal' component that decides the direction from which the most light is received by the attached polygon corner. This is also represented by a vector: the default is
(0 0 1), which means "full intensity straight up".