The Native Image Generator, or simply NGen, is the ahead-of-time compilation (AOT) service of the .NET Framework.
This eliminates the JIT overhead at the expense of portability and disk space; whenever an NGen-generated image is run in an incompatible environment, .NET Framework automatically reverts to using JIT.
This makes it possible, for example, to use NGen to process CLI assemblies at installation time, saving processor time later on, when the end-user invokes the application on their system.
NGen is intended to make the assembly execute faster by removing the JIT compilation process at runtime, but this does not always improve performance because some optimizations can be done only by a JIT compiler (e.g., if the JIT compiler knows that the code is already running with full trust, it can skip certain expensive security checks).
Because of this fact, it makes sense to use NGen only after benchmarking the application performance before and after it.