First rem, em is a unit of length, and px, following describes the concept of various units alone:
px pixels (Pixel). Relative length unit. Px is a pixel with respect to the display screen resolution. PC terminal with more and more.
em is the relative length unit. Text objects relative to the current font size. If the current line of text in the font size is not artificially set, relative to the browser's default font size, like the parent level.
rem is just opposite the root element HTML