An argument is any value that is being passed to a scripting command or an executed script or function. In the script or function this value is considered a parameter.
Example:
[variable, 5, "string"] execVM "myScript.sqf"
variable, 5 and "string" are called the arguments given to the script. You could even go as far to call the Array containing these values the single argument.
Access[ | ]
In the script the argument can be accessed using the magic variable _this.