The Opening Tab is where you will enter the coins that you start with at the beginning of the year.
For example, if you ended 2017 with 5 BTC, you would enter 5 BTC into your 2018 Opening Tab.
Keep in mind, if you are using BitcoinTaxes for multiple years, your Opening Tab will be automatically filled with the data from your previous years. You can also add it using the "Import from YYYY" button on the Opening Tab, as seen below:
