In Advance Wars each unit fights with a Weapon. Different units have different Weapons and each kind of Weapon has it's own seperate effect. Not every unit will have a weapon, non combatant units such as APCs are obvious, though other units such as Black bombs do not have a weapon either.
The Main Weapon (AKA Weapon One or Primary Weapon) is a units first Weapon and so is generally the units specialty Weapon. Not every unit has a Main Weapon, they may have a secondary weapon, or no weapons at all. Main weapons are the only kind of weapon to use ammunition and are generally the stronger than secondary weapons.
Each Main Weapon (and only Main Weapon) has a certain amount of ammunition. One round is used each time the unit attacks or counter attacks. Once the unit runs out of ammo, they cannot attack with their main weapon, though they will be able to attack with their secondary weapon if they have one. Ammunition can be restored by resting in a city or base (land units only), port (naval units), or airport (aircraft). Ammunition can also be resupplied by friendly APCs and black boats.
The seconday (or Weapon Two) is a unit's second weapon. All Secondary Weapons have infinite ammunition, but these weapons are generally weaker. Some units (like infantry) only have a secondary weapon and no main one.
Range (Or Fire) is how far units Weapon can hit. The range of an indirect unit is denoted by two numbers (e.g. 2-3, 3-5, 2-6), and the first of these digits indicate the unit's lowest possible firing distance from its target, in terms of squares, while the second number indicates the farthest possible distance of the unit from the target that it will fire.