The Barbarians in this context is simply another term for the Persians.

Barbarian literally meant anybody who didn't speak Greek: the entire anti-Persian coalition spoke various dialects of Greek, so the Barbarians cant refer to any of them. And while the Ionian Greeks inside the Persian kingdom appear to have contributed substantially to the Persian war effort (such as Artemisia of Halicarnassus), it seems likely (considering the scale of the Persian army that Herodotus presents) that he saw the Persian army as being largely Persian-speaking, or at least not Greek-speaking. It might seem strange that he says Barbarians and then changes to Persians and vice versa, but Classical writers didn't really think about this: to Herodotus, the two would have been synonymous in the context.