The Mine singer elaborated on the feelings that went into the tune, which is rumoured to be about her ex Taylor Lautner.
Swift talks about the tune in a new special, Taylor Swift: Speak Now, which is set to air on Thursday in the US.
The 20-year-old country singer said: 'The song Back To December is an apology in the form of a song.
'You know, in life, you learn lessons and sometimes you learn them the hard way, sometimes you learn them too late.'
She added: 'The song Back To December is about a lesson I learned too late and I think, when you do that, and you finally realise that you were wrong, you need to say you're sorry.
'And this was the best way that I knew how.'