An adverb meaning "each/any/all/every" — basically "no matter which one"; with negative verbs it means "none" (not any).
このケーキはどれもおいしそうで選べない。
All of these cakes look delicious; I can't choose.
どれも簡単な問題だから安心していいよ。
They're all easy questions, so you can relax.
どれも彼の説明には納得できない。
None of his explanations are convincing.