It's a glitch which allows you to -- under certain circumstances -- purchase weapons or armor from a shopkeeper, then sell them back for more than you paid for them, and to continuously loop that process, thereby giving you anything you want from the shop, as well as all the shopkeeper's caps, essentially for free.
This YouTube video demonstrates it on PC, and I've used it successfully in both Fallout 3 and Fallout New Vegas (already!). By the end of my last Fallout 3 run, I had over six figures in caps. Also, for a guy with less than an hour into New Vegas, I think I'm doing OK financially -- almost 800 caps.
The trick to it is having in your inventory a weapon or piece of clothing/armor which the shopkeeper sells, preferably at a lower repair value. Sell it to him, and commit the transaction. Buy it back and commit. Lather, rinse, repeat. It may take two or three repeats before you start buying it at its original value but selling it as 100% repaired.