This quote actually comes from a column that Poe wrote for The Columbia Spy entitled "The Doings of Gotham." You may find it helpful to note something about the context, as this will only help to shed further light on the quotation that you have highlighted.
Poe was writing about the trial of Polly Bodine, and, in bemoaning the inability of the police to investigate crimes successfully, Poe remembers the affair of Mary Rogers and the way that the police were particularly influenced by the press and what they published about the case. In particular, Poe finds it incredibly frustrating that the police (and the press) only focussed on the immediate factors, whilst completely ignoring the circumstantial and collateral events, which, Poe believed, were so important for this case. Thus Poe said this famous quote, that points towards the way that truth has more to do with the irrelevant than what we assume to be the seemingly relevant. Poe argues that our perception of truth and how we view it needs to change in order for us to understand it and comprehend it. We need to look beyond the easy answers and what we feel is relevant to deeper factors that we normally would not associate as being relevant.