Breakdown of code for referencing data in closed workbooks

I have 6 workbooks that have data I want to refer to in formulas in 1 central workbook. I can do this but the workbooks all have to be open.

I need code for doing this without having to open all workbooks.

I am pretty new to writing code so if anyone can do this and break it down for me that would be great.

Thanks in advance folks.