How do I check my inventory in Sumtracker?

Open Stock Levels table from the menu. You will see the following screen

Here is an explanation of each of the columns in the table - 

In Stock - This represents the actual (physical) stock currently in your warehouse

Booked - Quantity committed in open orders, not yet shipped

Available - Free inventory that is available for users to buy

(Available Stock = In Stock - Booked Stock)

Incoming - This shows how much quantity of this sku you have put on purchase order. This quantity is due to arrive from your supplier.

If you want to add stock, refer to this page.