Upon boot, the dmesg output is from the kernel booting, showing the devices it has found and if it has been able to configure them at all (aside from userland configuration). This log is also available in the file /var/log/dmesg.
The program helps users to print out their bootup messages. Instead of copying the messages by hand, the user need only:
dmesg > boot.messages
and mail the boot.messages file to whoever can debug their problem.
You can use
dmesg | more
If you're getting error:
dmesg: read kernel buffer failed: Operation not permitted
you can run as root user:
# echo 0 > /proc/sys/kernel/dmesg_restrict
to relax dmesg buffer reads ( kernel compile option CONFIG_SECURITY_DMESG_RESTRICT )