What is the difference between Warehouse stock and Store Stock?

They are two separate counts:

• Warehouse stock is the quantity of each SKU held in each warehouse.
• Store Stock is the quantity on the shelf at each Store Location (a shop or pickup point). When you assign a SKU to a store, you can note its Warehouse (source).

Online orders reserve stock from warehouse stock, from the warehouse with the most units of that SKU. The stock count shown on the storefront is also the warehouse total. Store stock is not touched by online orders.

Both pages work the same way: Assign SKU, then + In / − Out to adjust, and History to see every change. Assigning a SKU to a store does not take units out of the warehouse. To actually move units, use Inventory → Stock Transfers.