Taylor has said that this song is about the “red flags” she has seen in a person before their relationship. In that sense, the heartbreak is largely her fault because she knew exactly what she was getting into.
When someone puts a “notch in their belt” it means that they have added another person to the list of people they have dated.
He moved on from their relationship very quickly and it is evident that all she will be be to him is a name on that list.
This video is processing – it'll appear automatically when it's done.