Probably a dumb question… but I'm blonde - please forgive me!

I can remember seeing a spreadsheet that had a lot of little + and - (plus and minus) signs down the left hand side that could be clicked on to expand and minimise data

I think this could be helpful to me - but I can't remember what this function/facility called so that I can find it in Excel and also to research help on forums etc

Please can anyone help me by telling what the name of this is that I am talking about?

Many thanks in advance

Rae