Marshaling is the process of creating a bridge between managed code and unmanaged code; it is the homer that carries messages from the managed to the unmanaged environment and reverse. It is one of the core services offered by the CLR (Common Language Runtime.)
Because much of the types in unmanaged environment do not have counterparts in managed environment, you need to create conversion routines that convert the managed types into unmanaged and vice versa; and that is the marshaling process.
As a refresher, we call .NET code “managed” because it is controlled (managed) by the CLR. Other code that is not controlled by the CLR is called unmanaged.