To password protect access to your phpBB forums.

1. Go to the "Forums" tab in the Admin Control Panel

2. Now you will see categories. See tutorial to learn the terminology between category, forums, topics, and posts. If you want to edit the permission for the whole category, you can click the green edit icon...

Or if you want to edit the permission of the individual forums within the category, click into the category link so that you see all the forums...

For this example, we'll click the green edit icon for the forum to set the permission of the forum.

3. Note that depending on the whether you are setting passwords for category or forum, the "forum type" (shown below) will be either be "category" or "forum"...

Enter forum passwords that you want to set.

4. Click submit button at the bottom. Now that forum will require that password. Or if you have set the password to a category, then all forums within that category will require the password.

Note: this refers to version 3.0.5 of phpBB.