An “em” is a printer’s measure of width: the width of the letter “m” in some print font or the other (and I don’t actually know which font). You can probably get a fairly detailed dissertation by using google.
I *think* (I am most probably wrong though) that as regards wp, an em in your theme is set by the base font shown in the body section. YMMV, and someone more knowledgeable will no doubt be along shortly to correct me, and point fingers, laughing….
I believe vkaryl has it right, except that an “em” isn’t the size for some specific font, it’s for whatever font you are using. So, the size of “11 em” will change depending what font you decide to use. If you want specific sizes instead, you can change “em” sizes to pixel (“px”).