The Default Site for your account is the one that will be available at the root domain of your account. For example, if your account domain were
my-company, the default site would be available at
Also, if you were using the ScreenSteps Google Chrome Extension, entering my-company in the Account Name field when logging in would connect you to the default site on your account.
You probably shouldn't change your default site. If you do change it, be aware that any incoming URLs that were pointing to the old site will break.