Roleplaying: Acting, or Self-Insert?

I've seen roleplaying defined as a few things, and as the key to having fun and being immersed.

There's the common trope of acting; you disappear yourself inside another character.

Then there's simply playing out how you'd act if it was basically you with a few differences.

I feel like most people just say "roleplaying" when they mean one or the other, which can create some misunderstandings. Which one are you supposed to be doing? When a new player asks "how do I play?" what are you supposed to say?

Roleplaying: Acting, or Self-Insert?