The CSS (Cascading Style Sheets) style code (look at the page source), for this file defines the default behavior of the P tag to have 5em of indentation. The "EM" unit is a printer's term meaning the width of a capital letter M. Thus, all paragraphs in the page will be indented by 5em. This turns out to be a lot. You can also specify the indentation in pixels, such as 50px. An em is a useful unit of measure because it then varies by the size of the font that someone uses: 3em in a 24point font is more distance than 3em in a 12 point font.

This paragraphs in this file are an example of the usage. The paragraphs are indented, meaning that the first line is begun at some distance from the left margin of the paragraph.