I believe taxable sales is sales amount of sales for those items you set as taxable.  Just the price of the item without sales tax you collected from the customer.(or you should have.. because that is what DOR is waiting for you to send it to them)  Total collected is price of the items that you sold plus the sales tax amount you have Set up... plus sales amount of non taxable items.(if you also sell non taxable items)  

For example in Kent Washington sales tax is 10%.  So if your total collected is $100 and tax is showing $3 on your sales summary report.  Your sales of taxable was $30... sales tax you collected for it is $3.. and your non taxable sale is $67. So 

Your total sales is $97.. because $3 sales tax is not part of your sales