For those who are wondering at the rationale behind this: many adverbs end in -ly. "The brown fox quickly ran", "she urgently asked", "he mercilessly ate his salad", etc.

Adverbs should be avoided, because, as danso notes, you must "show, not tell". When the brown fox "quickly" runs, you're telling the reader what happened. Similarly, when she "urgently" asks, you're not showing the reader what's going on.

Instead, you can describe what's happening with better verbs: "the brown fox sprinted after his dinner", "she pleaded for the answer key", "he attacked the tomato in his salad".

That's just one way to do it, of course. You can also describe the action in detail: "After skewering his tomato, Jessie brought it up to his mouth, where it tumbled into his esophagus".