adverb, noun which may take the genitive case particle 'no'
that much; to that extent; only that; that is all
Expresses degree, quantity, or limitation. Can mean 'to that extent' or 'only that much/no more than that'. Also used at the end of a statement to mean 'that's all I have to say'.
それだけあれば十分です。
That much is enough.
彼の成功はそれだけの努力
His success is the result of that much effort.
以上です。それだけです。
That's all. That's all I have to say.
Refers to 'this much' (close to the speaker), while それだけ refers to 'that much' (close to the listener or previously mentioned).
Refers to 'that much' (distant from both speaker and listener), while それだけ is more immediate or contextual.
Particle meaning 'only' or 'just'; それだけ combines the pronoun それ with だけ to mean 'only that' or 'that much'.
Compound of それ (that) and だけ (only, extent). The derivation is transparent.