It's the ratio of width vs height. You can reduce it to a fraction. 640 x 480 reduces down to 4 x 3 as in the screen is 4 parts wide by 3 parts high. 2560 reduces down to 16 and 1440 reduces to 9. Same aspect ratio at 1920 x 1080 (1080p) and 1280 x 720 (720p).
1080 refers to the height of the screen in pixels (vertical resolution) and the P stands for "progressive scan" as in each successive row of pixels is refreshed in order, as opposed to "i" for "interlaced" where every other row of pixels is refreshed..