http://www.gimp.org/tutorials/Sketch_Effect/

Copyright © 2004 Vidar Madsen

Meet Marius, my son. :)

High pass filtering means that we filter away the low frequencies of something, and let the high frequency bands pass. In image terms, this means that the detail of an image is kept, while the larger scale gradients are removed. Luckily, it's not as complicated as it sounds.