Sentence:  You can specify the type size below which text will be <WSD>greeked</WSD> in the Preferences dialog box. Designers often prefer to use <WSD>greeked</WSD> text in rough layouts because it helps the client focus on the design rather than on the words. 
Meaning of the word marked with <WSD> tags:  To display a placeholder (instead of text), especially to optimize speed in displaying text that would be too small to read.