The screen door effect is the name given to the undesirable visual artefact produced by tiny lines separating pixels and/or subpixels in displays. It is particularly an issue for displays on head-mounted devices, as they are positioned close to the viewer and magnified by lenses. The effect is named for being similar to looking through a screen door, as seen here:
Not only does the screen door effect harm image quality, but it also harms colour, reducing brightness and vibrance by masking the image with darkness.