json vs html in rails

have some question when generate controller appear some comment

i want to know use importance with these line

# GET /folusers # GET /folusers.json def index

first question rails generate json file. can use instead of api version .

# POST /folusers # POST /folusers.json def create

second question how access create method by json

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final question need to create mobile app and connect with db are enough use generate in controller or use api version ?