How should i understand "Stored up kamma"?
It's somewhat sloppy language... "stored up potency" doesn't really mean anything; it's a concept, the concept that certain experiences will somehow be related to other experiences. There is nothing in Theravada Buddhism, AFAIK, even hinting at karma being stored up, or of a continuum being imprinted.

So, it should probably be understood as conventional language, similar to explaining how a seed from one tree makes a new tree - the potency of the first tree is stored in the seed, creating a tree continuum. In ultimate reality, the continuum doesn't exist, nor the potency, the tree, or the seed.