"what's that on your wrist?"
"I'm a cutting board. duh"
"what's that on your wrist?"
"I'm a cutting board. duh"
The joke plays on the double meaning of 'cutting'. One person asks about the cuts on the other's wrist, implying self-harm, but the other person responds as if they are literally a 'cutting board'. This unexpected and dark response is where the humor comes from.