Welcome to the Cecil wiki!
In simple English, with Cecil, you can load existing managed assemblies, browse and inspect all the contained types, modify them on the fly and save back the modified assembly.
If you want to compile Cecil, you can read the Compilation guide, and if you want to contribute, you want to read the Coding Conventions page. If you’re looking at porting code that uses Cecil 0.6, you should read the Migration guide.
An easy way to use Cecil from your application is to get the Mono.Cecil NuGet package.