When scientists or researchers field-test something, they try it or test it in the environment in which it's intended to be used.
Engineers, developers, and inventors all have reason to field-test products. For example, if a drug company is developing a new medicine, they might first test it in a laboratory and then field-test it, having actual patients try it. Educators might field-test a new curriculum, and military planners often field-test weapons or vehicles before outfitting entire armies with them. In field-test, the field is the "natural location or context."