A really fast thesaurus... with lots of synonyms.
Strong verbs replace flat ones with specific action: instead of walk use stride or trudge, instead of run use sprint or bolt, and instead of look use gaze or glare.
Strong verbs name a specific action — stride, sprint, devour — instead of a vague one like walk, run, or eat, so the reader pictures exactly what happened without needing an adverb.
A precise verb does the work of a verb plus an adverb: "sprinted" is sharper than "ran quickly". Strong verbs make prose shorter and more vivid at the same time.