3D models are build from points in 3D space where each point has a x, y and z component. Points can be connected with lines (vertices). Connected lines can form a face (polygon).
Here you see two base models (primitives) with two different view options. On the left the wireframe view, which shows you the lines, and on the right the flat solid view, which shows you the faces. Each line connects two points. As you can see a point can be shared by multiple lines.