The output of frament processing is to figure out the target pixels and their characteristics. However before these pixels are written to the framebuffer to become part of the screen they have to undergo number of tests. Does the pixel position in the framebuffer is currently owned by this OpenGL context or Is that controlled by someone else. If the depth testing is on, is this pixel behind another pixel? Is there a stencil active that may block this pixel from showing? These are are called fragment tests. If the pixels don't pass the test they won't be written to the frame buffer.