I'm not sure specifically what 'Pretty Little Letters True Story' is about without more context. It could be a story related to some beautiful or interesting letters, perhaps in the context of a mystery where the letters hold some crucial truth.
It depends. Sometimes a story labeled as such might draw inspiration from real events but have fictional elements added. So, it might not be a purely true account.
As I'm not familiar with this particular story, it's hard to provide examples. However, if it's a love - centered story, an example could be a letter where one person writes 'You are the most beautiful thing in my world' to their beloved. This kind of letter would be part of the 'Pretty Little Letters' that tell the true story of their relationship.
It isn't a true story. Usually, works like this are crafted to engage the readers' imagination and provide an interesting fictional narrative rather than representing real-life events.