True stock is actual stock available for new orders.
Regular stock will show everything in the system including inventory reserved for on-hold and unfulfilled processing orders.
Let's say you have 50 items in regular stock but only 30 in true stock. If you put a new order for 50 of that item, it will go into exceptions. Since 20 of the items are already reserved for other unfulfilled orders, you would need to change the quantity to 30 to move it to processing.
How can I view True vs. Regular stock?
Regular stock can be viewed in the products page (Inventory > Products) and is listed as "ON HAND"
True stock can be accessed in the dashboard in the inventory status page (Inventory > Inventory Status)
There is a column for IN STOCK (regular stock) and a column for TRUE STOCK