The defs section in the JSON contain all project components definitions: layers, tilesets, entities and their fields, etc.

"defs": { 
	"layers": [ ... ], 
	"entities": [ ... ], 
	"tilesets": [ ... ], 
	"enums": [ ... ], 
	"externalEnums": [ ... ] 
}

Important note: most of these definition data won’t be really useful for games, as the useful parts are duplicated in double-“_” prefixed fields (see above). They are a mostly used by the Editor app itself.