A monitor's resolution describes how many pixels it uses to generate the picture, commonly referenced in Width:Height format eg. 1920x1080P meaning 1920 pixels wide and 1090 pixels tall. As pixel count / density increases, higher levels of detail become possible to display. 

There are a number of fairly common monitor resolutions such as 1920x1080p (referred to as Full HD or "FHD"), 2560x1440p (Quad High Definition or "QHD"), and 3840x2160p (Ultra HD, "UHD," or "4K").