You need to click on Publish button when you want to make the following changes:
App color theme;
Content of the tabs and elements (phone number, email, text, Url etc…) and their order in the app;
The name of tabs and elements (icons)
Using the “publish” feature is a much faster option as it sends out updates to apps immediately without having to do an app store submission.
You need to click on the Resubmit when you want to change:
Splash screen, icon;
Name of the app;
If you add new elements (tabs/buttons) which are not supported by your current app version (new functionality since the last version of your app) you will get the message telling you to Resubmit your app.