yes, you can "password protect" some function ( navigating to a new frame. loading a new swf, etc)
various ways with various levels of security.
an easy way, that is not terribly secure, is to have an input text box serve as your password container. a simple button click can verify using an 'if' conditional statement if the password is correct. if it is, it allows the user to do whatever yo want them to do.
i wouldn't use the technique for financial or other strict security situations, but for entrance to a 'vip access area' it could do the job easily
well, certainly don't use that method where financial or other PPI could be at risk.
you might note the big flaw in this usage - that the password itself is stored in the SWF itself. so while this would be a good means to keep the honest people honest, it doesn't protect from those intent on doing harm.
anyone who could get hold of the swf could theoretically decomplie it and expose the code ( which contains the password ). you could of course encrypt your swf to protect it from decompliers which would add a large measure of safety.
but still, for full out serious protection, you would need to access a secured connection back to a secured server. hopefully someone else can offer insight in how to best go about doing that.