Facebook checks to see if the endpoint provided is valid or not.

FacebookVerify.py

@app.route('/', methods=['GET'])
def verify():
    # when the endpoint is registered as a webhook, it must echo back
    # the 'hub.challenge' value it receives in the query arguments
    if request.args.get("hub.mode") == "subscribe" and request.args.get("hub.challenge"):
        if not request.args.get("hub.verify_token") == os.environ["VERIFY_TOKEN"]:
            return "Verification token mismatch", 403
        return request.args["hub.challenge"], 200

    return "Hello world", 200