Something that is explicit is stated directly and is clear in meaning. Explicit meaning is the easiest to pick out from a text.
Sometimes a writer wants it to be obvious that the atmosphere of a text is good, bad, dangerous, happy, sad, and so on.
Something that is implicit is inferred - it is suggested by the way it is said. Implicit meaning can be harder to figure out than explicit meaning.
Writers like to be clever and draw you into a text. It is up to you to make your mind up about a setting, in the same way that it is up to you to make your mind up about a new place you visit.
Writers often use language in effective ways so that the description of their settings also helps to create a suitable sense of atmosphere and mood.