The following are all absolute length units — they are not relative to anything else, and are generally considered to always be the same size.
Most of these units are more useful when used for print, rather than screen output. For example, we don't typically use cm (centimeters) on screen. The only value that you will commonly use is px (pixels).
Relative length units are relative to something else, perhaps the size of the parent element's font, or the size of the viewport. The benefit of using relative units is that with some careful planning you can make it so the size of text or other elements scales relative to everything else on the page. Some of the most useful units for web development are listed in the table below.
MDN : https://developer.mozilla.org/en-US/docs/Web/CSS/position