Hiding Columns and Rows

Hi I am aware of right clicking on a row or column and then clicking on hide, but I came across a file that was entirely beyond anything I have ever encountered before.

Would it be possible for somebody to explain to me how to create excel sheets such as the one I have attached?

Thanks!

It was just the fact that somebody hid so many rows that had me a little confused. I'm used to the columns part, but it's that there are so many rows gone I thought that it was some other trick.