Don't Understand What This Code Block Is Calling From Spreadsheet

The attached screenshot is from the AddPipes.dyn that comes standard when Dynamo is installed. I am having trouble understanding what the code block with a[0] is pulling from the spreadsheet? This is probably something simple, I just can’t find any info/notes online. Thanks

In the list of 1st item. Same as get item at intex at the intex of 0