OK well lets break it down.
A weaponholder is an invisible ammo crate. It cannot be created in the mission editor: usually they are created in init.sqs but you could do it in a script, trigger or waypoint if you really need to. Lets say that you wanted to create a weaponholder with a fully loaded kozlice, at the position of a marker called "WH" and with the weapon pointing in a particular direction: you would write this code in your init.sqs
weaponH1 = "weaponholder" createVehicle getMarkerPos "WH"
weaponH1 addMagazineCargo ["kozliceball", 4]
weaponH1 addMagazineCargo ["kozliceshell", 4]
weaponH1 addWeaponCargo ["kozlice", 1]
weaponH1 setPos getMarkerPos "WH"
weaponH1 setDir 70
In this case it's named weaponH1 but you can call it anything you like. If you put in one weapon then the thing you will see in the mission is that weapon: if you put only magazines in it (no weapon) then you will see a satchel-like object on the ground, the same as the object you see if you drop a magazine while playing a mission. (I'm not sure what happens if you put more than one weapon in, I think you see the first weapon added.)
Does that help? Just try it and see.
Another example:
weaponHolder2 = "weaponholder" createVehicle getMarkerPos "base"
weaponH1 addMagazineCargo ["M16", 4]
weaponH1 addWeaponCargo ["M16", 1]