In the Full .NET framework you had the concept of native images generated using Ngen.exe(Native Image Generator). By creating a native image, you can improve the startup time and performance of managed applications. A native image contains compiled processor-specific machine code. This image is then installed into the native image cache on your local computer. The runtime can use native images from the cache instead of using the just-in-time (JIT) compiler to compile the original assembly.
With the upcoming release of .NET Core 3.0, a similar feature will appear in .NET Core through ReadyToRun(R2R) image. R2R is a form of ahead-of-time (AOT) compilation.
R2R binaries improve startup performance by reducing the amount of work the JIT needs to do as your application is loading. The binaries contain similar native code as what the JIT would produce, giving the JIT a bit of a vacation when performance matters most (at startup). R2R binaries are larger because they contain both intermediate language (IL) code, which is still needed for some scenarios, and the native version of the same code, to improve startup.
In contrast to NGEN where compilation must be done on client machines, R2R images are generated using crossgen, as part of your build and are “ready to run” without any additional work on client machines.
For more details and some performance benchmarks, read more here: https://devblogs.microsoft.com/dotnet/announcing-net-core-3-0-preview-6/