Where you're not just dumped in and you have to figure everything out yourself. And where you are not given tons of information at once in quick succession.
But it's also not as slow as a mobile freemium cash grab tutorial, where things that are obvious to gamers are pointed out and made to be the only thing you are allowed to interact with.
Saints Row 3 does a very good job of introducing game mechanics.
It sometimes falters by dropping an overly hard optional mini-game in the player's path too early without clarifying that it is optitonal.
Otherwise, nearly a perfect game to play through and unlock interesting mechanics in a relatively smooth progression.
The "crew" mechanic does a very good job as an optional any-time challenege slider. When I'm having trouble, I can "phone a friend". (also works with actual friends - the multiplayer is great and drops into the same mechanical gameplay slots).
Saints Row IV also does this really well - but... it's really different. It jumps the shark early and then keeps jumping back over the shark, over and over again. If that sounds fun, you'll love it. But it's not everyone's cup of tea - and the game mechanics can come in uncomfortably fast if the player gets lucky in finding some unlocks before being prompted to seek them out.