A callback URL is a URL that is invoked after OAuth authorization for the consumer (connected app). In some contexts, the URL must be a real URL that the client’s web browser is redirected to.
Callback URL in salesforce is nothing but what Salesforce will callback with once the user authorizes access to his/her Salesforce account. Callback URL is typically the URL that a user’s browser is redirected to after successful authentication. You have enter call back URLs one per line.