The sequence diagram example shows three participating objects: Customer, Order, and the Stock. Without even knowing the notation formally, you can probably get a pretty good idea of what is going on.
Step 1 and 2: Customer creates an order.
Step 3: Customer add items to the order.
Step 4, 5: Each item is checked for availability in inventory.
Step 6, 7, 8 : If the product is available, it is added to the order.
