This is an account window. You control your set of accounts from here.

Below is a picture of an account window, with only the main accounts shown. Note how Business Expenses has been selected by highlighting. The box with a cross in it, immediately to its left, shows that it has one or more detail accounts that are not shown at present. To show them, click on this box.

Here is the main window, with the detail accounts showing.

The Open Subaccounts menu item is interesting only if you choose an account with sub-accounts (detail accounts). Accounts with sub-accounts will always have a box on their left. By choosing the Open Subaccounts menu item, a general ledger window is opened.