A particle indicating limitation: "only", "just", or "merely" — restricts the scope to what is mentioned.
ケーキは一切れだけください。
Please give me just one piece of cake.
彼だけ来なかった。
He was the only one who didn't come.
昨日は水だけ飲んだ。
Yesterday I only drank water.
Used to indicate extent or sufficiency: "as much as", "to the extent of", or "enough to" (e.g., 'できるだけ' = 'as much as possible').
できるだけ早く来てください。
Please come as early as possible.
あと千円だけあれば買えるのに。
If I had just another 1,000 yen, I could buy it.
見るだけで違いがわかる。
You can tell the difference just by looking.