The joke here is that 煎れる (roast/brew) is read ireru, the same as 入れる (insert). "Brew it strong" and "put it deep inside" are homophones.
Dark roasts have a strong bitter taste and the flavor is strong.These numbers should be labeled in the store...Light roasts have a strong acidic flavor but is otherwise mild.Acidity If you guys like it bitter, then......should just pick whichever you like.Bitterness ...make sure you roast it hard, okay?Pun with "put it in hard"