Images of Text
If the technologies being used can achieve the visual presentation, text is used to convey information rather than images of text, except where a particular presentation is essential (such as a logotype) or the image can be visually customized to the user's requirements.
In plain language
Whenever it's possible to show text as actual, real text, don't use a picture of text instead.
Why it matters
Real text can be resized, have its color changed, and be read by screen readers. A picture of text is frozen — it can't be resized cleanly, doesn't adapt to a user's custom colors, and screen readers can't read it unless someone remembered to add a text description.
In practice
Fails this criterion
A page uses an image of styled text for a heading, when the same visual style could easily be achieved with real text and CSS.
Meets this criterion
The same heading is real text styled with CSS to look identical — it can be resized, recolored, and read aloud, exactly like any other text on the page.
Test it yourself
Try selecting the text on the page (click and drag to highlight it) and zooming in. If something that looks like text can't be selected or gets blurry/pixelated when zoomed, it's probably an image of text — and unless it's a logo, this criterion is failing.