How to measure results?

I'm soon gonna hire an SEO company but I have no idea what their rep is talking about in the business meetings. (That's why I'm here to try and learn about it)

Is there an secure and trustworthy way of measuring their work? Like this site is ranked 6/10 because of ....?

Since I don't know what I'm buying really, is there even a good way of paying a fixed commission for result? Idk , kinda struggling here but all websites and people around me is saying this is really important.