What is rooting? How to root an android phone? Explain what is rooting? How to do it? Does rooting affect the phone's performance? Can samsung galaxy fit be updated to present android version?
can anyone refer sites that explains the inside of android?
Did you tried Google Android rooting?

Is there something in the articles that your don't understand?

Rooting means allowing an application to gain root privilege. This is usually done by hacking the existing kernel or flashing a new kernel.

Some devices locked the bootloader, you have to "unlock the bootloader" first in order to flash a new kernel.

As for updates, enthusiasts port the latest AOSP to various devices. Xda forum is a great source for these custom ROMs (and rooting/flashing info).

WARNING: Rooting, unlocking bootloader and flashing custom ROMs may void your warranty.