It is a file, which sets some rules for Visual Studio debugger and instucts it to visualize JsonCpp (https://github.com/open-source-parsers/jsoncpp) objects in a special manner. With such visualizer reading of the debugging JSON values became much easier, because of compact data representation.
I've tested it with jsoncpp 0.6.0-rc2 on Visual Studio 2015 SP3 and currently have no information could it be run on other versions.
How to install
There are several ways to deploy natvis file. The easiest one is to copy jsoncpp.natvis to the %USERPROFILE%\My Documents\Visual Studio 2015\Visualizers. You have to create Visualizaers folder if it does not exist.
Other methods are described in the following article: https://msdn.microsoft.com/en-us/library/jj620914.aspx#Anchor_3