it's saying that the past makes you who you are "and those little things define us forever" but you shouldn't look back on the past too much "I don't wanna hear about the bad blood anymore" and you shouldn't let your past haunt you "all this bad blood here, won't you let it lie"
there are other quotes I could use to back this up, but I'm too lazy to get into it
so in a general statement, the song is about moving on from the past
so basically this song is about the past
it's saying that the past makes you who you are "and those little things define us forever" but you shouldn't look back on the past too much "I don't wanna hear about the bad blood anymore" and you shouldn't let your past haunt you "all this bad blood here, won't you let it lie"
there are other quotes I could use to back this up, but I'm too lazy to get into it
so in a general statement, the song is about moving on from the past