5K describes a display resolution of 5120 x 2880. Resolution explains how many pixels a display has in length x width format (the more, the better when choosing a PC monitor).
Few monitors actually have this resolution today, though Apple's 27-inch iMac offers it. LG's 27-inch Ultrafine 5K monitor also outputs at 5120 x 2880. Few games benefit from running at this high of a resolution. Unless you're a graphics professional, you can probably be satisfied with a lower resolution screen.
Common Monitor Resolutions:
|5K||5120 x 2880|
|4K||3840 x 2160 (typical monitor resolution)|
4096 x 2160 (official cinema resolution)
|Ultra HD (UHD)||3840 x 2160|
|Quad HD (QHD) aka Wide Quad HD (WQHD)||2560 x 1440|
|2K||2560 x 1440 (typical monitor resolution)|
2048 x 1080 (official cinema resolution)
|WUXGA||1920 x 1200|
|Full HD (FHD) aka 1080p aka HD||1920 x 1080|
|HD aka 720p||1280 x 720|