だけ is used in Japanese in a similar way to ‘only’ or ‘just’ in English. The basic meaning of this word is that ‘while there are/were several possibilities, only (A) is true/was chosen’. だけ is used by attaching it to the base (dictionary) form of any word, except for な-Adjectives, which require な when being used adjectivally.

• 今夜（こんや）は食（た）べるだけ、何（なに）も飲（の）まない。
  Tonight I'm only going to eat, I won’t drink anything.
• 私（わたし）はブロッコリーだけがきらい。
  I only dislike broccoli.
• あの人（ひと）は有名（ゆうめい）なだけ。
  That person is just famous. (Fame is all they have)
• このスープは辛（から）いだけ。
  This soup is just spicy.

Due to な-Adjectives being able to be used as either a noun or an adjective, the meaning of a sentence will be different depending on whether だけ is used with the noun form, or the adjective form.

• 私（わたし）の彼女（かのじょ）は綺麗（きれい）なだけです。
  My girlfriend is just beautiful. (There are no other distinguishing features)
• 富士山は綺麗（きれい）だけじゃない。
  As for Mt. Fuji, beauty is not all there is. (There are other distinguishing features)

Basically this difference just highlights whether something is being discussed in a measurable way (an adjective), or an immeasurable way (a noun).