only; just; as much as
だけ marks the limit of what is included, meaning only or just that thing and nothing more.
だけ is a particle that limits the scope of what you are talking about. If you say これだけ, it means only this thing, not anything else. You can put it after nouns, verbs in their plain form, and adjectives. For verbs, the plain form can be present or past: 食べるだけ (only eat) or 食べただけ (only ate). For な-adjectives, you use the plain form: 静かなだけ means 'only quiet' or 'merely quiet' (e.g., 静かなだけの部屋). For い-adjectives, you use the plain form: 高いだけ (only expensive). Note that だけ can also mean 'as much as' in a different pattern, like 欲しいだけ (as much as I want), but the core meaning is 'only.'