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 of using the just-in-time (JIT) compiler to compile the original assembly.

you can refer this:
https://learn.microsoft.com/en-us/dotnet/framework/tools/ngen-exe-native-image-generator