(tentative definition until I find more secure references)
Davidson's field linguist is a figure used to illustrate his theory that, in any language, most of the statements formulated will be true. Thus, a field linguist who lands on a polynesian island and attempts to learn the language of the native tribe will do best to assume that most speakers are making positive, commonsensical assertions, and to develop his knowledge of the new language coherently, rather than correspondantly.
In other words, the field linguist must try to make sense of the language in terms of itself, and not by trying to develop a language-neutral dictionary or manual of translation; the latter would lead to forcing the native language into the field linguist's mother tongue, which would lose the sense of usage within the native tongue.
Davidson generally follows Wittgenstein's assertion that usage determines meaning in language, not correspondence to the world.