That's pretty simple: you should type in the same value
in the Confirm field, as you already provided in the above field.
You don't have to confirm it that way:
In the future releases of miniBB, on the Registration form the Password confirmation option will not be offered anymore. Instead, an unmasking/revealing option will be offered, which should bring less frustration and prevent any errors.
* Login fields on the front page
* Login fields below the posting form on each of the Topic pages
* The Admin login area
* Registration/Profile form
* For the Private forums — on the initial common login form to all forums.
As many debates and research studies shown, repeating/confirming email or password would lower the form's conversion rate. In most often cases, these values are copied/pasted anyway, that's why it wasn't clear initially of why they are needed.