"Fearless concurrency" yes. In Rust's model we can't make a lot of the easy mistakes. Safe Rust won't let you, for example, try to share an Rc<T> between two threads, because while Rc<T> is perfectly sound and has good performance locally, if it's possible for two concurrent operations to touch the object bad things may happen, so we can't ever (in Safe Rust) see such an object from more than one thread - in this case you should use the (more expensive on some hardware) Arc<T> if there might be more threads.

In safe Rust understanding why all this works isn't your problem, it just does. Hardcore implementation work (say, you're making a new kind of mutual exclusion primitive) will need unsafe and also rely on proper understanding of how to (and whether to) enable use of your type this way.