The fact that it is an imaginary [..] world means that it is somehow set apart from the “real” (or “Primary”) world, with some boundaries between them, making the secondary world a thing of its own; and whereas some boundaries are physical or geographic in nature, such as mountain ranges, deserts, oceans, and so forth (or the surface of the earth itself, for underground worlds), some boundaries are temporal in nature (as in worlds set in the distant past or future, making them equally inaccessible to us in the present), or even conditional, such as in the alternate versions of the Primary world that some stories present. Tolkien separates the two by calling them the Primary world and Secondary worlds (borrowing terms from Coleridge’s discussion of the two types of imagination), and writes that the latter is dependent on the former, hence the term “subcreation” (literally, “creating under”); secondary worlds use material from the Primary world, reshaping and recombining elements from it, so that the end result is both recognizable but also new and different.
Building Imaginary Worlds: An Interview with Mark J. P. Wolf (Part One).