Through which values does this sum iterate $\sum_{i_1,...,i_k = 1}^{n}D_{i_k}....D_{i_1}f(x+t\zeta)\zeta_{i_1}...\zeta_{i_k}$ mean?

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What is the first summand, the second summand and so on? How many sumands are there. I am sorry if this question is too short. I hope you can still help me.