I assume that it is for chrome. I have read that I can set the permissions in that file to avoid XSS errors while developing.
There are many json files for your chrome experience from Native chrome json files to json files for chrome extensions. Most of them can be found at
C:\Users\username\AppData\Local\Google\Chrome\
Most manifest.json files are found for extensions in
C:\Users\username\AppData\Local\Google\Chrome\User Data\Default\Extensions