How do I explain this haircut to a. Barber

I'm still searching around for a good barber. And I want to make I specify what I want properly before I ask them to cut and I don't want to judge the work based on not giving the proper instructions.

I'm M27. I have very straight thick hair. What do the various numbers mean when referring to trimmers? I have always kept my hair long for most of my life . And I want to try this out to look younger. The kindof frameless spectacles and long hair I have make me look like a guy in his mid 30s