Definition
The C# compiler reads your scripts and writes a managed assembly that contains IL plus metadata describing every type and member. The compiler keeps your names, string literals, and the structure of each method, because the runtime and the debugger rely on them. With IL2CPP, a C++ compiler then turns generated C++ into a native binary for the target platform.
Relevance
The compiler is what turns a Unity project into something a player can run. It keeps the names and the shape of each method, because the runtime, the serializer, and the debugger all rely on them.
Example
You write a method named ApplyDamage that takes an int. After compiling, the assembly contains the name ApplyDamage, the parameter, and the method as IL. That assembly is what the player loads.