Designing for readability means styling text with the reader’s comfort and comprehension in mind. When building websites (or anything else made of text) we should choose fonts, colors, text sizes, text spacing, and column layouts that are easy on the eyes and on the mind. Sometimes prioritizing readability also means compromising your aesthetic vision for the greater good.
Readability has a related but separate meaning referring to the clarity of a text in terms of how it is actually written and edited, rather than how it is displayed. FWIW that’s not what this site is about.
r9y is a numeronym (the first and last letters of a word with the rest replaced by their count). I propose it as a shorthand for the concept of designing for readability, like the numeronyms for sibling concepts a11y (accessibility) and l10n (localization).
Defining a shorthand for readability as a design principle is especially important because the commercial service Readability™ has (understandably) chosen to co-opt the word as a trademark for their brand, confusing its meaning and confounding search engines and design newcomers alike.