Justified text is spaced so the left and right sides of the text block both have a straight edge. Justification works by adding white space between the words in each line so all the lines are the same length.
If you’re using justified text, you must also turn on hyphenation so you don’t get gruesomely large spaces between words. Unfortunatly hyphenation is not widely supported in internet browsers which is why justified text is usually discouraged on the web.
Read more about this in this article about Justified Text