In desktop publishing, to display text in a representative form in which the actual letters are not discernible, because the screen resolution isn't high enough to display them properly. The software lets you set which font sizes should be greeked.
The term comes from typography and graphics design, in which Greek or Latin words are used in a mockup. They hold the position and represent the real text that will be forthcoming. Foreign symbols are used so that the text can be quickly identified as fake.
Greeking
These pages are greeked in this print preview mode, because there is not enough resolution on screen to display them correctly.