Question

I can’t really understand what these modes are doing.

I see you set
glMatrixMode(GL_MODELVIEW);

before you draw vertices, but what does that actually mean? What is it doing?

Answer 1

In OpenGL there are two main matrix modes (there are two others, but you probably don’t need to be concerned with them), they are GL_MODELVIEW and GL_PROJECTION. Together they define much of how opengl transforms the vertices you put in into the visuals you get out. When you have called glMatrixMode(GL_MODELVIEW) most recently, you will be altering how your points will be shifted around in space, you can cause all of your points to be translated with glTranslatef, rotated with glRotatef and so on. When you have called glMatrixMode(GL_PROJECTION) you will be altering how your points get projected onto the screen. This is the mode you want to be in to call things like gluPerspective or glOrtho.

It is common to switch to projection mode before each major drawing step (main scene, GUI, etc) set up the projection matrices, then switch to modelview until the next one. This way, you can leave your projection unaltered while you change around the transformations of the scene by moving the camera, altering positions of objects, etc.