what changes are done, while rooting an android phone?

You have added a binary named su to your phone's operating system file. Android is based on Linux OS. Not all users can do everything they want in Linux. For example, an app in your phone, can't access to another app's data. It doesn't have the necessary permissions as a normal user. All apps you install are assigned a UID (User ID) by the system. There is however, another user in Linux OS that can execute all commands and can access everywhere and has all permissions. That is the root user (UID 0). When you root your phone, you add a binary and an app to manage it. (usually SuperSU) When you grant an app root access, it can execute commands and have permissions it normally can't. So beware of the apps you grant root access to.