If I told you that dmesg is probably a contraction of the words 'driver message', would that help? The linux kernel uses drivers to perform many tasks. When a driver has something to say, it does so by emitting messages which the syslog facility collects, and which the dmesg tool allows users to view. Most messages are emitted at boot time, when the drivers are initializing themselves and hardware. It is possible for drivers to emit messages at any time.