Greeking

If text gets smaller than this value, [Adobe] Illustrator shows the text blocks as gray bars, an operation called greeking. Both type size and view size figure into the equation, so that 6-point type greeks at 100-percent view size and 12-point type greeks at 50 percent. Greeking speeds the screen display because gray bars are easier to draw than individual characters.

Source: wiktionary

Designers have long worked with a concept known as greeking. Greeking is a method of drawing gray bars to represent blocks of text rather than taking the time to image them all on the screen. […] Unless you really want to see every character of every word at every size, there's hardly any reason to turn greeking off.

Source: wiktionary

The use of greeking (also called dummy text) allows page designers to place text on a page without the actual content acting as a distraction. The history of greeking is unclear, but it was first used in the middle of the 20th century. The text was based on the words of Cicero, a Roman leader whose writings are admired. The language he spoke and wrote was Latin rather than Greek, so the naming of this dummy text is misleading.

Source: wiktionary