Colour displays on smartphones and PC monitors use a grid of coloured dots, or pixels, which are themselves each made from red, green and blue subpixels. A standard RGB display typically uses one subpixel of each primary colour to create each pixel, varying the intensity of each to create the full range of displayable colours.
The PenTile system uses a different layout with more green subpixels than red or blue, reducing the total number of subpixels required for a given display resolution and relying on image processing to reconstruct the full-resolution image. In LCD panels, a different PenTile technology is used, adding a white subpixel to the matrix that can allow for significant power savings as more of the backlight can be allowed to shine through. Technically, PenTile displays offer lower image resolution than standard RGB ones, but in modern HD displays this difference is rarely perceptible.