Hz (hertz) is the number of times a panel can refresh per second. So a 144Hz monitor can have 144 separate images on screen per second. Higher hertz makes images smoother. 4k is 3840 x 2160, or UHD, QHD is 2160 x 1440, and FHD is 1920 x 1080, and I think people call that 2k?
Edit: so when saying 1080p, 1440p etc you use the second number, or the vertical number of pixels, and 4K/2K is by the first number, or the number of horizontal pixels.