Herodotus' main goal was to collect knowledge through inquiry (ἱστορία, historia) . He didn't always have the means to really examine it, though he would sometimes demonstrate a preference when given more than one option. Some of this would come from works written by others before him, local knowledge (e.g. "the people of this specific place say..."), common knowledge (e.g. "the Greeks say..."), speaking with other people he considered to be holders of knowledge (e.g. priestly class), and access to people who either lived during/through events or knew those who did. There's not always a clear indication of why he chose to discuss certain things and why he leaves some things out (sometimes painfully so), but sometimes those alternatives survive through other sources and he does occassionally note that something is already known in an essentially "so why would I bother to tell you?" sort of way.

While it has become less popular to do so, there were attempts in modern history (and some in antiquity) to dismiss what he did because of a lack of critical evaluation, but considering what was available to him, when he wrote it, and the scope of the writing it's a monumental and greatly important work.