noun
Used to refer vaguely to things without specifying them, similar to 'this and that' or 'such and such' in English. Often used when the speaker does not want to or cannot be specific.
これこれの理由で、今日は休みます。
For such and such reasons, I'll take the day off today.
彼
He said he does this and that kind of work.
あれこれ means 'this and that' in a more general sense of various things, while これこれ is used when referring to specific but unnamed things, often in a list-like manner.
Reduplication of これ (this), used to indicate multiple unspecified items.