Hi Tanushree,
Perhaps you could look into following articles and video as they have a good explanation on package.appxmanifest. Hope this helps you.
1. AppxManifest File in Windows Store Apps
http://www.c-sharpcorner.com/UploadFile/99bb20/appxmanifest-file-in-windows-store-apps/
2. WP8.0 vs WP8.1 - All About New Package.appxmanifest File at Beginners Level
http://www.c-sharpcorner.com/UploadFile/55275a/wp8-0-vs-wp8-1-all-about-new-package-appxmanifest-file-at/
3. Configure a Windows 8.1 app package by using the manifest designer
https://msdn.microsoft.com/en-us/library/br230259.aspx
4. Working with the package.appxmanifest
https://channel9.msdn.com/Series/Windows-Phone-8-1-Development-for-Absolute-Beginners/Part-8-Working-with-the-package-appxmanifest
Thanks,
Sabah Shariq