ELI5 Masterpassword please

I set up Biometric Lock on my Proton Pass on my Android phone. But after opening the app, if I cancel the fingerprint unlock, it asks for me Masterpassword.

Now this is where it gets slightly confusing. I tried, BOTH, my account and my mailbox passwords - none of them work. And it says I have 1 more attempt left.

I definitely don't remember setting up any type of Master Password.

Is it that master password is just blank, and no matter what i type in, it will be "incorrect"?

So what is master password? It's a third password?