I’ll try to explain. Remember from L1 how the BS, IS and CF statements are integrated? For example:
Cash received by clients = Revenue - change Accounts receivable + change Unearned revenue
Cash paid to suppliers = COGS + change Inventory - change Accounts payable
Cash paid for other operating expenses = Other operating expenses + change Prepaid expenses - change Other accured liabilities
So basically if you put all these changes in operating assets and liablities on the one side, on the other you will have:
NI - actual cash received / paid for operating and investing activities.
So this change in NOA is the part of net income which is not supported by cash. As we know this part is mean-reverting i.e. if your profit is not supported by cash most likely it will decline in future.
I hope that helps.