It's a style of talking through irony, meaning the opposite of what someone says. Example1: While browsing at the shop, your "skinny" friend looks at an XL t-shirt; and you mockingly say: "Wow, that'll look so nice on you". (Obviously, you knew your friend was just looking due to the size). You could just offend your friend by saying: This is so big for you because you're skinny. But you wanted to be nice, so you've decided to be "sarcastic" by saying: "Wow, that'll look so nice on you.", which simply means the t-shirt is too big for your friend. Example2: During a very hot summer day, you suggest to buy some ice cream for everyone at home. Suddenly, your "brother" jokingly says: Can we get some hot cappuccino as well? Then, you say: Are you being sarcastic? (Are you kidding me?) Of course, your brother was being "sarcastic". ( He was was just kidding). I hope it's clear to understand 😀