This is the Showing forth of the Inquiry of Herodotus of Halicarnassos, to the end that neither the deeds of men may be fortotten by lapse of time, nor the works grat and marvellous, which have been produced by Hellenes and some by Barbarians, may lose their renown; and especially that the causes may be remembered for which these waged war with one another.In other words, he is bound to record what has happend so that future peoples would know and not forget. He will write about both the Greeks and the non-Greeks and he will give special focus to the causes of war. In short, he will write history.
The great thing about his approach is that he recognizes that some of what he has been told is true, and some isn't. He sifts through these categories and shares his judgment with the reader. If there are differing versions of stories, he'll mention that. In fact, Herodotus had actually traveled to several places and he uses first hand knowledge in his writings.
All of these things set the template for the writing of history. Modern historians will listen to ancient stories as a starting point, but then they work to figure out what the truth is. They'll study conflicting records and travel to locations to unearth evidence. They'll also maintain a level of skepticism until they have proof. All of this follows the discipline that Herodotus set down.