OpenGL drawing shapes

currently i'm doing a simple paint drawing coding. my knowledge in OpenGL is quite low
I don't really understand the glVertex2d

Like i've attached the code and when i do the checking of the x and y cordinates with my mouse, it displays x = 211
y = 300

how isit link with the -1.0,-1.0?? is it due to the window size?
how do i convert it from 211, 300 to -1.0, -1.0??

and i'm drawing shapes using the mouse like a paint program, do i use glutDisplayFunc() to apply layers??

glBegin(GL_POINTS);
  glVertex2d(-1.0, -1.0);
    glEnd();

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