If a file's attribute is marked immutable it cannot be modified. we chattr for changing attribute.
root@desktop:~# rm temp
rm: remove write-protected regular empty file `temp'? y
rm: cannot remove `temp': Operation not permitted
root@desktop:~# ls -l temp
-rw-r--r-- 1 root root 0 Oct 28 04:26 temp
root@desktop:~# lsattr temp
----i------------- temp
"temp" is immutable (i flag)
root@desktop:~# chattr -i temp
root@desktop:~# rm temp
root@desktop:~# ls temp
ls: temp: No such file or directory