The Native Image Generator (Ngen.exe) is a tool for the .NET framework that improves the performance of managed applications by creating and caching processor-specific machine code, so that just-in-time compilation is no longer necessary.
From Native Image Generator (Ngen.exe) on MSDN:
The Native Image Generator (Ngen.exe) is a tool that improves the performance of managed applications. Ngen.exe creates native images, which are files containing compiled processor-specific machine code, and installs them into the native image cache on the local computer. The runtime can use native images from the cache instead using the just-in-time (JIT) compiler to compile the original assembly.
