When accessing a system you generally provide an “Identity”. This is usually a unique identifier to say I am “X”. A password is simply a system to prove that identity.
You want access to your bank account, to do this you need to prove to the system you are who you say you are. Simple.
If someone else wants to access you bank account, they need; your identity and your proof of identity.
Getting your “identity” can be relatively simple; your name, your bank account number or identifier, an email address etc
Next is figuring out the password.