It means to specify font color, font background color, a specific font, or a specific (rather than relative) font size. In practice, this means that you should not make your entry's presentation beautiful in a word processor and then paste it in to the rich text editor, and you should not pick color, background, font, or point size in the rich text editor.

You're free to use <em> </em> and <strong> </strong> for emphasis, and if you need fine print, <small> </small> is the best relative way to do it.

The reason for this is as princessofgeeks said: it means that all your choices get forced for everyone else trying to read it, rather than the choices they've made with their own style for their reading page. There are a lot of people who need their specific setup for accessibility reasons -- low vision, migraines, eye strain -- and if you have a hard time with the community style, you can apply your own journal's style or the site skin you picked by using that option from the bar at the top.