Dueling DQN what does a' mean?
It is just a type of namespacing, because $a$ is already assigned the chosen action. There are two contexts of action being considered in the equation, so there needs to be a symbol for each context. Using $a'$ is an obvious choice as the letter $a$ is implicitly linked to representing an action already.

The sum over $a'$ is a sum over all possible actions in state $s$, irrespective of the chosen action $a$.

So both $a$ and $a'$ represent actions. $a$ is the current action, supplied on the LHS of the equation. $a'$ represents the iterator of a sum over all actions $[\forall a' \in \mathcal{A}(s)]$, only used in the calculation on the RHS. Sometimes you will see a completely different letter chosen, or some subscripting or other way to show these represent different actions.

It is also quite common to see $a$ representing current action, and $a'$ representing the next action (taken when in state $s'$). But that is not what is happening here.