A Property Tree in Boost is a data structure that can store information in a tree-like format. Each node having a key and a value. It can be used to store XML, INI, INFO and JSON formats. Maybe also other, but these are the most popular and they are supported by boost, with ready-to-use parsers.
The limitation for JSON handling, is that property trees do not support simple arrays, since all nodes are of key-value type.