Flooding is a behavioristic psychological tool for reducing anxiety. As opposed to desensitization, where increasingly powerful versions of the anxiety-causing simulus are given to the patient, while teaching him to dissasociate the stimulii from anxiety, in flooding, the patient is flooded at once with the stimulus. Usually at maximum potency.

Thus, in the desensitization of arachnophobia (fear of spiders), the patient would be first subjected to thoughts about spiders, then to pictures of spiders, then to seeing spiders in a different room, then close up, and finally touching a spider. In flooding, the person would basically be given a few spiders to play with, and after a little while, he would see it's not too bad, and hey presto, the fear / anxiety is gone!

I am, of course, slightly simplifying things here. This is just the tip of the iceberg of these techniques, but those are the basics.

For more reading, I would suggest cognitive / behavioristic therapy guides, or maybe even just Introduction to Psychology books.