Businesses that reorder products based on fixed reorder thresholds can use reorder points. Naturally, reorder points are defined per product as a company has different thresholds for different products. Whenever the quantity on hand of a product falls below the reorder point, then that product is in a state of needing reorder.
To bring stock levels up above the reorder thresholds, a company can create reorders for all products in the state of needing reorder. Products may be purchased from different suppliers, so the reorder process often involves making reorders from all of the suppliers relevant to the products in the needing reorder state.
A reorder itself is just a purchase order with values filled in for the supplier and for the items and order quantities. If you want to adjust the order quantities, you can simply change the numbers in the order. The idea of the reorder process is to give you the default quantities to reorder in an automated process that you don't have to think about.
Example from Finale Inventory
Here is a video showing reorder points and reorders in Finale Inventory.