ICWYDT means "I 'see' what you did there."
'I see what you did there' is commonly used as a sarcastic response, as it is a clever turn of phrase.
Person 2: Ha. ICWYDT.
Person 1: Dude, can you go into your parents room?
Person 2: Uhhh... yeah, why?
Person 1: Because I need you to get my pants back from your mom.
Person 2: ... ICWYDT