What is actually happening on the rom when I rooting android device? What is happen on rooting?

When you use your phone, access applications and navigate it's file system, you do so as a user with a given set of permissions. This places certain restrictions on what you can and can't do or access.

Rooting your phone places an executable on your device called su (switch user), this executable switches the account credentials and permissions that you use to interact with the file system on your phone, making you a super user.

Android is based on the Linux Kernel, and this is essentially the same as achieving 'root' on a Linux device (hence the term rooting).