Re-rolling is a crafting-related term meaning that you repeatedly craft the same recipe with the horadric cube. This is usually done when there's motivation to achieve a certain kind of item, usually because its commonly desired affixes by market, but also because one player may be in search of an item with very specific statpoints.
Re-rolling is oftenly practiced by rich and often more dedicated players, due to it oftenly being very time consuming. Not only in the active part of the process (meaning put the ingredients in the cube and hit transmute), but also when farming/trading for the massive amount of ingredients needed. Then organizing these big stocks of packages onto mules/crafting-character and after the craft get rid of the worse results and store the others.
It is also of high importance to be very knowledgable with the mechanics of crafting in general, and to have subjective knowledge of what is "good" stats versus "bad" stats and so on.