this song is about an ex lover that after the relashionship was over he started to ignore the girl and it came to the point were he had his friends go to her house and take his stuff nad then afterwords he even changed his number, this is saying that he didnt have to do that or even go that far and even if his actions did hurt her she sees him as somebody that she used to know.

But you didn't have to cut me off
Make out like it never happened and that we were nothing
And I don't even need your love
But you treat me like a stranger and I feel so rough
No you didn't have to stoop so low
Have your friends collect your records and then change your number
I guess that I don't need that though
Now you're just somebody that I used to know

the affect on this song is imagery because i visualize a girl being sad while her ex lover passes by her and ignores her completely as if they were strangers.