Nickel Metal Hydride (NiMH) batteries naturally get quite warm while charging
, unless you use a specialized "slow charger". If your phone uses a NiMH battery, do not worry about the heat generated during charging unless it becomes so hot that it is uncomfortable to touch.
mAh is an abbreviation for milli-ampere hours which are units of electrical charge. Larger values, for batteries of the same battery voltage, indicate that the battery has a larger capacity and will power your phone for a longer period of time before charging.