an argument is a constant or variable that you provide to a function; e.g.
sin(x) % here x is the argument to the function sin.
The command line is the line in the command window where you type commands. After you type the command, you hit "enter" and then Matlab evaluates the command and displays the answer. Usually the command line has ">>" at the beginning; so if I type "sin(x) followed by <enter> on the command line, Matlab will calculate sin(x) and display the answer.
>> sin(0.1) <enter>