Usually done every month to adjust all inventory transactions amount as per model group attached to each item like: FIFO, weighted average etc.
Before going to process please read for item model here. Example FIFO: An Iphone X is purchased $1,000 as original price in the month of JAN 2018 with 1 QTY. Again purchased in the month of JUNE 2018 with price $999 then in DEC $800. Assume no transactions of any issue in year 2018, Year 2019 first issuance is done then as per FIFO the transaction is adjusted as $1,000 first in first out when closing is run.
Note: Once closed that period and before this period the inventory transactions will be locked however it can be reversed also until year end close is done permanently.
Inventory Management->Periodic->Closing and adjustment
The generic way is re-calculation and closing. However additionally we can see open quantities, items which cannot be settled hence gets carry forwarded until receipt is matched with issue. The unmatched are hence open quantities items, read here.
Item Code C0001 is purchased with PO no PO/0006 with qty 6 -> receipt done->Invoice done->Later consumed all the 6 quantities (by Production Order, Adjustment, movement or project item journal)
Again Item Code C0001 is purchased with PO no PO/0006 with qty 3->receipt done->Invoice Not done->Later consumed all the quantities (by Production Order, Adjustment, movement or project item journal)
This consumed -3 quantities will look for invoiced qty which is not done and pending hence system will show -3 quantities are open
Closing in detail see here.