In one context, for example, $\mathbb{Z}_5$ would mean a set $\{0,1,2,3,4\}$ with the usual additive arithmetic modulo $5$ defined on it.
Similarly, $\mathbb{Z}_5^3$ would mean a 3-dimensional set of vectors, each element of which comes from $\mathbb{Z}_5$, so
$$
\mathbb{Z}_5^3 = \left\{ (a,b,c) | a,b,c \in \mathbb{Z}_5\right\}
$$
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It probably means $\mathbb Z_5 \times\mathbb Z_5 \times\mathbb Z_5$, where $\mathbb Z_5$ means $\mathbb Z/5\mathbb Z$.
In one context, for example, $\mathbb{Z}_5$ would mean a set $\{0,1,2,3,4\}$ with the usual additive arithmetic modulo $5$ defined on it.
Similarly, $\mathbb{Z}_5^3$ would mean a 3-dimensional set of vectors, each element of which comes from $\mathbb{Z}_5$, so $$ \mathbb{Z}_5^3 = \left\{ (a,b,c) | a,b,c \in \mathbb{Z}_5\right\} $$