beginning openl

I am starting opengl and I understand everything in the code below except for how they got the coordinates I understand that when you start off by defaul you are looking down the z axis, but how did they know har far to translate and the coordinate numbers themselves??

Code:

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#include <GL/gl.h>
#include <GL/glut.h>
#include <GL/glu.h>
#include <unistd.h>
#include <stdlib.h>

int window;

void Display(void){
        glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
        glLoadIdentity();
        glTranslatef(-1.5,0.0,-6.0);
        glBegin(GL_POLYGON);
                glVertex3f(0.0,1.0,0.0);
                glVertex3f(1.0,-1.0,0.0);
                glVertex3f(-1.0,-1.0,0.0);
        glEnd();
        glTranslatef(3.0,0.0,0.0);
        glBegin(GL_QUADS);
                glVertex3f(-1.0,1.0,0.0);
                glVertex3f(1.0,1.0,0.0);
                glVertex3f(1.0,-1.0,0.0);
                glVertex3f(-1.0,-1.0,0.0);        glEnd();
        glutSwapBuffers();
}

void keyboard(unsigned char key,int x,int y){
        usleep(100);
        if(key==27){
                glutDestroyWindow(window);
                exit(0);
        }
}

void reshape(int Width,int Height){
        if(Height==0)
                Height=1;
        glViewport(0,0,Width,Height);
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        gluPerspective(45.0,(GLfloat)Width/(GLfloat)Height,0.1,100.0);
        glMatrixMode(GL_MODELVIEW);
}

void InitGL(int Width,int Height){        glClearColor(0.0,0.0,0.0,0.0);
        glClearDepth(1.0);
        glDepthFunc(GL_LESS);
        glEnable(GL_DEPTH_TEST);
        glShadeModel(GL_SMOOTH);
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        gluPerspective(45.0,(GLfloat)Width/(GLfloat)Height,0.1,100);
        glMatrixMode(GL_MODELVIEW);
}

int main(int argc,char **argv){
        glutInit(&argc,argv);
        glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGBA|GLUT_DEPTH|GLUT_ALPHA);
        glutInitWindowSize(640,480);
        glutInitWindowPosition(0,0);
        window=glutCreateWindow("Yellow Ledbetter");
        glutDisplayFunc(Display);
        glutFullScreen();
        glutIdleFunc(Display);
        glutReshapeFunc(reshape);
        glutKeyboardFunc(keyboard);
        InitGL(640,480);        glutMainLoop();
        return 0;
}

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basically, in that tutorial, they use 45 degrees because the tangent of 45 degrees is 1, that means that if you move down the z axis (actually, you are wrong about this, gl by default uses right hand coordinate system, negative z axis you are looking down, x axis off to right) by 1 unit then that means that your top and bottom boundaries are also bounded by 1 unit (this is because that gluperspective function makes 45 degrees the fovy, to solve for fovx you multiply by aspect ratio).