Purchase orders let you record orders placed with suppliers, track their status (New, Released, Received, Closed, Cancelled), and update quantities when goods are received. You can also see from a product’s detail page whether that product is on order and when it is expected.
Section: Top menu Warehouse > Purchase orders, or go to Purchase orders.

From the list you can:
Orders move through these statuses:
When you close an order, warehouse stock is updated according to the quantity received (outstanding reduced to zero for those lines). Product quantities and stock history reflect the received amounts.
For each order line you can set the quantity outstanding: the amount still to be received. When you partially receive goods, reduce the outstanding quantity and save. When everything is received, set outstanding to zero and then close the order so that stock is updated.
To see if a product is on order and when it is expected:
Section: Products > Click a product > Purchase orders tab.

The tab shows only open orders (not closed or cancelled) and only lines that still have quantity outstanding > 0. For each line you see:
This data is loaded only when you open the Purchase orders tab, so product details stay fast.