As used here it is so far a mere metaphor, and it is necessary to explain directly the concept which it is meant to indicate.
Durations can have the two-termed relational property of extending one over the other.

Thus the duration which is all nature during a certain minute extends over the duration which is all nature during the 30th second of that minute.

This relation of 'extending over'-- 'extension' as I shall call it--is a fundamental natural relation whose field comprises more than durations.

It is a relation which two limited events can have to each other.

Furthermore as holding between durations the relation appears to refer to the purely temporal extension.