mah is milli amp hours. It's the capacity of the cells. More mah means more run time. If you only used one 2s LiPo battery, it would only be 7.4 volts nominal. The E-Maxx is set up to run on 2 packs in series which gives you 14.4(NiCd, NiMH) or 14.8v LiPo. If you get 2 6000mah LiPo packs run in series with a dual brushless setup, you'd probably get 25 to 30 minutes of runtime. If you step up to the 12,000mah packs from maxamps, you'd get bored with the truck before the batts went soft.

Tony