The OpenGL function glClear requires one parameter, which is an OpenGL constant such as GL_COLOR_BUFFER_BIT. The constant tells OpenGL which buffer (or buffers) we want cleared. The color buffer (sometimes called the frame buffer) corresponds to the video memory on a graphics card that holds the pixel-color information being displayed on the output device. Clearing the color buffer means filling the buffer with the current clear color, which is established with a previous call to glClearColor or is the default clear color black. Filling the frame buffer with a clear color before rendering our geometries makes that color the background color of our image.