well as you can see, i clearly have no idea what i am doing buuuuut...im looking at 4wheelparts.com to get some new tires. there are having a deal, buy 3 ,get 1 free tire from bfg. they have two 31" tires, and they are 31/10.50R15 (Ply Rating=8 Tread Width=8.1 O.D.=30.2)
PCT15031, and 31/12.50R15 A/T
PCT15231. what do the numbers after the "31/" mean, and which one should i get? thanks.

The number after the 31 (which is the diameter in inches) is the section width of the tire at its widest point. The 12.50" wide tire will be too wide to fit the Xterra without a suspension lift and body lift.

the 10.5's should be fine on a stock xterra though? (btw, what do stock xterra's use)

Yes, the stock tires are similar in width, but in a metric size notation (like 235mm wide).