"Blocking" means to wet or steam the knitted item and then laying it out to straighten the edges and manipulate it into the exact measurements you want, and then let dry completely. Yarn is softer when wet and when dry, it will stay in the shape you put it in. Much like your hair... if you wet your hair, then put it in curlers, when dry, it will still be curly.
And by the way, I don't use a blocking board either. I use a peice of cardboard with a towel over it.