I'm not sure specifically as there could be many things it could refer to. It might be about a series of letters that had some sort of malicious or naughty undertones in their history.
I'm not entirely sure without more details, but it might be significant in terms of character development if it's part of a story. Maybe the letters changed how certain characters behaved.
It's not a true story. 'Wicked Little Letters' is most likely a piece of imaginative writing crafted to captivate the audience and take them on a fictional journey.