How far a thing extends, and the act of extending toward. Both are about what can be touched from where you stand.
Old English ræcan (to reach out, to extend, to attain), from Proto-Germanic raikijan; reach names both the extending itself and the distance that extending covers, with no distinction made between the act and its measure.
How far a thing extends, and the act of extending toward something. Both senses share one root because what can be touched from where one stands was always, first, an act rather than a fact.