.JSON files are use in some STM32 applications to configure the behavior of it or some sw functionality.
For example in the package STEVAL-STWINKT1 in the HS_DataLog example the .JSON file is used for determinate what sensors logs and at what speed.
Here there are some .JSON examples for STSW-STWINKT01 (HS_DataLog example).
For more details concerning the .JSON file see on Internet.