The feeling you get when you haven't wiped properly after expelling faeces.
There are 2 main causes:
1. The article was passed in a semi liquid state potentially due to spicy food consumed previously.
2. You wiped prematurely smudging the stool into a cake and in the clean up process made your ass bleed by wiping to hard.
Someone with poo burn can commonly be identified due to the awkward way they are sitting, spreading their crack and rubbing it down the edge of the seat to gain relief and simultaneously shit stain their own undergarments.
Steve: "Man, I wiped early while taking a shit, It was everywhere and I didn't get it all out. I even made myself bleed, now I've got real bad Poo Burn "
Dave: "Dude, stop rubbing your fucking ass down the seat"