What does if the chain is on your door i understand mean?

First of all , If the chain is on you door means if your door is locked. That is to say that she understands that he may doesn't want to have a relationship with Taylor or even talk to her...

Back to December? best song :)

it basically means, 'if you will never let me in to your heart again, I deserve it and understand where you are coming from.'

How you cant understnad this? :O

she means a gift from her to him^^

and I'M from German