You're not supposed to put something there...
The first line explains the bind command - or how to create the trigger.
The second line explains what will be executed when the binding triggers.. That is, assuming we've got this:
This creates a "/msg" binding for the word "Hi", and when it triggers it will call the proc "do_hi" with for parameters: the nickname of the user, the user@host of the user, the handle (as your bot sees it) of the user - or * if unknown, and whatever the user wrote after "Hi".
As such, your proc (do_hi) would have to accept 4 arguments (one for each parameter). The first argument will hold the first parameter (the nickname), the second argument the second parameter, etc... Each argument will then be available as a local variable with the same name within the proc.
What you call these arguments is pretty much up to you (as long as you don't use "args", as this is a special argument name for tcl), although using descriptive names usually helps writing good code. A common practice is "nick host hand text", but you could just as easily use "nick userhost handle text", or simply "n u h t". It doesn't matter, as long as you keep track of the names of the arguments/variables in your code...
In your first example, you don't make use of any of the arguments, so the names are really not important, only that you have one for each parameter (total of 4).