Basically the "Stack AI Land" option gives the AI a 5-10% improvement although this is just a guess. You can use this option to make the computer a little harder or easier. If I am playing MTG Forge's quest mode and the computer's decks are too easy I turn this option on and if I am losing more often, I turn it off. You can also do the same thing for sealed and draft.
Specifically the "Stack AI Land" option gives the computer 7 spells and 1 land. The computer then draws lands for the next 3 turns, then a spell, and then 3 more lands (I think). This improves the AI because it gives the computer 7 spells to choose from for the first couple of turns. A future improvement could stack the computer's deck with an optimal mana curve so the AI would always have something to play for each turn.
p.s. When the computer has to discard a card, I coded the AI to just discard a card at random. While this solution isn't perfect, it works "good enough".