The Unity C# Job System allows users to write multithreaded code that interacts well with the rest of Unity and makes it easier to write correct code.
Writing multithreaded code can provide high-performance benefits. These include significant gains in frame rate. Using the Burst compiler with C# jobs gives you improved code generation quality, which also results in substantial reduction of battery consumption on mobile devices.
An essential aspect of the C# Job System is that it integrates with what Unity uses internally (Unity’s native job system). User-written code and Unity share worker threads. This cooperation avoids creating more threads than CPU cores, which would cause contention for CPU resources.
For more information, watch the talk Unity at GDC - Job System & Entity Component System.