The only thing I can think is that the person who is judging is only scared of seeing his/her own faults and tries to comfort his/herself by telling others they are doing right or wrong or just critizising for all we know. We get tired of that and fight back that's why she says "next time you point a finger I'll point you to the mirror" what I think she means is if you've got your own faults don't judge 'cuz you're not perfect either. My whole perspective of the songis if you got faults then don't critizise or judge "cuz chances are you aren't perfect either.