Woah! This is a big one. And let us just say that handing over the passcode to a phone is the same feeling for both genders. So, imagine you're giving her your passcode and that's how much of a big deal it is. Of course, if you two are in love with each other you know you're not going to go snooping through each other's messages, but you've got the capability to, and that's the significance here.
The fact that she's giving you her passcode also means that she has stopped talking to any other potential love interests. We all play the field a little until we're sure we're with a good one, so this is a sure sign that you've wiped out all of the competition. Keep her trust and don't rifle through her phone unnecessarily.