what is the meaning of Kernel \r on an \m
 

/etc/issue file contains an entry
"Kernel \r on an \m" along with the OS version and build.
Eg:

cat /etc/issue

Red Hat Enterprise Linux Server release 6.1 (Santiago)
Kernel \r on an \m

see man issue: The file /etc/issue is a text file which contains a message or system identification to be printed before the login prompt.
and see for example man agetty (ISSUE ESCAPES) about meaning of:
r Insert the release number of the OS, eg. 1.1.9.
m Insert the architecture identifier of the machine, eg. i486