What Are Litotes?
Litotes are a specific type of understatement where instead of expressing something in a straightforward and often positive way, a negative term or terms are used to create emphasis.
Instead of the affirmative sentence, 'My friend Briana is funny,' imagine if I said, 'My friend Briana is not unfunny.' The first expresses the sentiment in a simple way, so there is no denying that she is indeed funny (which she is). It comes off as a compliment: 'My friend Briana is funny.'
On the other hand, the second version with litotes says something different. With 'She's not unfunny,' it's like I'm saying that she barely has a sense of humor, and with the changing of a few words, a compliment becomes an insult.