Look inside and you will see
The words are cutting deep inside my brain
Thunder burning, quickly burning
Knife of words is driving me insane
Insane, yeah
Raven black is on my track
He shows me how to neutralize the knife
Show to me in surgery
The art of fighting words to conquer life
Conquer life, yeah
[ooooh, ooooh, ooooh, ooooh]
Now the wicked lance of fear
Is driving from my heady mountain brain
Crash course in brain surgery
Has stopped the bloody knife of words again
Yeah, yeah, yeah [ Lyrics from: http://www.lyricsmode.com/lyrics/m/metallica/crash_course_in_brain_surgery.html ]
I think this song means that words can be like a knife driving itself into your brain, and the only way to stop it is to learn how to ignore the words, which would surgically remove the knife from your brain.