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

Booked (Outgoing) - As you receive the orders, their booked quantity goes up. This column represents how many units to this sku needs to be fulfilled

Available Stock (= In Stock - Booked Stock) - Available stock tells you how much stock will be available after fulfilling your open orders

On Order (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.