Need Help With Understanding Excel Program Command

Hello, all. I am new to this forum, as well as to Excel, itself, since I have never had a class on it, and live far from where any are taught; what little I do know has been from just trial and error, library books, and help from good people like you.

Background: I am trying to sort of "reverse engineer" a spreadsheet that was written by an internet friend with whom I corresponded a few years back. It is a spreadsheet that is used to process data obtained from an EEG device (electroencephalograph). Brain waves are obtained from the scalp via electrodes, then changed into a text file which can then be import into an Excel spreadsheet for visual analysis. My friend wrote the spreadsheet, then e-mailed it to me, so I could use it; however, I would now like to modify it a little, adding on a couple of worksheets. It seems to be a task which would be fairly easy to do, even for an Excel-challenged rookie, like myself; however, there are a couple of command/programming lines which I don't understand, hence my posting this question.

If someone would be so kind as to explain what each/all of the characters within the parentheses are for, I would appreciate it very much.
I'm sure that I could do so, but it would probably take quite a while, and I was hoping that someone who was a whiz-bang Excel guru could save me a whole lot of time and trouble, by taking a few minutes to explain what each of the characters within the command string does. I have ordered a couple of books re. Excel from the local library, and they should be in, after a few days, hopefully. I did that, in case I couldn't get any clear response from someone on this site.

Here's the line of programming that I would like to understand: =AVERAGE(INDIRECT(""&A2&"_Data!C"&start_window_6&":C"&end_window_6)).

I understand that the command directs Excel to return the average of the data contained within the parenthese, but I don't know how to decipher the rest of it.

Thanks, in advance, for any help that might/hopefully come my way.

I would guess that A2 contains a file name and start_window_6 and end_window_6 are named formulas that return row numbers. The formula would return the average of the cell range C?:C? on the worksheet Data in the workbook name in A2.

HTH
Rich