Ideally, I think it's like a master training you up to become a master yourself. I found the best treatment of this in Mastery. It's rarely an explicit thing these days and I think we're all worse off for it.

Few people are patient or humble enough to go through an apprenticeship phase and most jump to living their fantasy of being a master (assisted by all the internet's unverifiable broadcast mechanisms). They mostly end up slowing their rise to mastery, or preventing it entirely, because they don't want to live through a period where they are explicitly less-than.

I think I had a mentor at my first and only post-college job. He was more of a role model than anything. We'd chat once a week as I tend to be pretty hands-free, but occasionally we'd pair up if I was stuck on something obscure to me.

He was a true master and led the company's core networking protocol after leaving bittorrent. I really lucked out getting hired there as a reentry college student. The company was very keen on process minimalism, was founded and run by engineers, and prioritized dev talent. They eventually left me to my own devices and kept paying me more and giving me new responsibilities.

My other internship offer was at a now dead web startup and the hiring manager was a dick.

Before that, my high school math teacher was the best role model that I had. He was super contrarian and "saved me" from learning nothing in high school.