callback URL and Oauth 1.0

[Hanyka]

it is bit silly to ask but...

i read several times but...still cannot understand
why do we need callback URL while we are developing third party application.
what is it's function and why do we need it, if we only want to use that 3rd party app in my own phone only?

and..

last time , i remember that date as April 12th

i tried to get authentication for my 3rd party android app using oauth 1.0

and i followed those procedures to registered and in the end it keeps saying token expired.

expire date was show as "1970-~~" like this.

but last time , i checked one of topic's reply that says,

https://community.fitbit.com/t5/Web-API/OAuth-1-0a-removal-update/m-p/1272107#U1272107

the app which got oauth 1.0's authentication permanent token will be okay until August
 but no longer provide token or credential anymore.
and today, i checked there's no more oauth 1.0 page left on web currently.

did i got vaild token so far but can't use it ? or need to follow oauth 2.0 again? :S

[JeremiahFitbit]

OAuth 1.0a support has been removed for new applications. New applications should use OAuth 2.0.

Existing applications should upgrade to OAuth 2.0, as OAuth 1.0a support will be removed forever on August 1, 2016.

A "redirect URI" (OAuth 2.0 terminology) or "callback URL" (OAuth 1.0a terminology) is how Fitbit sends the user back to your app after the user gives your app permission. The redirect URI is also how Fitbit sends the access token (for Implicit Grant Flow) or the authorization code (for Authorization Code Grant Flow) that allows your app to make API requests on the authorizing user's behalf.