Actually glVertex parameters don’t relate to a screen position, but a vertex position in object space.
For instance, you want to draw a cube and its vertices coordinates are defined from its center. To do this, you move the center of the cube in world space where you want and then draw the cube giving its vertices coordinates with glVertex.
Now, in 2D, you have to set the viewport and projection matrix for orthographic projection and then draw vertices on screen with glVertex with only 2 coordinates. These coordinates are in the 2D space you set and can be in any range and not bound to [0 1].
See gluOrtho2D function reference page here.