A 'stretched out arm,' as in Exodus 6:6, signifies omnipotence or divine power, because 'an arm,' when stretched out in the heavens, represents power from the divine, but when not stretched out, but bended, it represents power in the general sense. This is why divine power is often shown in the Word as a stretched out arm or a strong hand.

(Reference: Arcana Coelestia 7205)

In Exodus 3:12, this signifies a confirmation of the truth that the Divine proceeded from Himself. (Arcana Coelestia 6870)

(Reference: Arcana Coelestia 1039, Genesis 9:12, 9:13, 9:17)