This is the 34th most frequent Japanese word.
だけ
"だけ" means "only" or "just."
だけ is used here to indicate exclusivity, meaning 'only', emphasizing that the person drinks juice and nothing else.
私はジュースだけを飲みます。
I only drink juice.
だけ is used here to indicate sufficiency, meaning 'just' or 'merely', emphasizing that simply looking is enough to understand.
見るだけでわかります。
I can understand just by looking.
だけ is used here to indicate a limit or degree, meaning 'as much as', emphasizing performing the action to the greatest extent possible.