dmesg displays the contents of /var/run/dmesg.boot file which is populated during the boot process with the white high intense master console messages you see rolling by during the boot which are the results from the motherboard hardware inspection [POST].
dmesg -a displays the the same as above plus the non-intense messages of started applications from /etc/rc.conf.
It can be helpful in tracking down the BDS hardware device names for thing like disk drives and network interfaces.
> dmesg [snip] ad0: 194481MB <Maxtor BAH41> [395136/16/63] at ata0-master UDMA133 ad1: 194481MB <Maxtor BAH41> [395136/16/63] at ata0-slave UDMA133 acd0: DVDR <SONY DVD RW DRU-500A/2.1a> at ata1-slave PIO4 ad1: WARNING - WRITE_DMA UDMA ICRC error (retrying request) LBA=3205439
> dmesg [snip] Mounting root from ufs:/dev/ad0s1a pid 6152 (audacity), uid 1001: exited on signal 11 (core dumped)