The 2 possible scenarios when your data will be indexed in any search engine's search. (i.e visible to whole internet) are:
* Your View [which contains the data entered in your application] is public.
* Your private View is embedded in a public website.
Your Form can be public.Even if indexed by search engines, it will not give out any sensitive data. By default, the application is private. To enable users to access your application, you'd have to share the application to the particular users alone. Refer this post for more details on sharing your application. Please note that your application and data are completely secure as long as it is private.