What is the internal representation of null in the memory when say we assign null to a string variable in Java?

Question

I have very high and limited knowledge of compiler's functionality.
As per my knowledge, Compiler does it's job using following steps:
1. Lexical analysis
2. Syntactic analysis
3. Semantic analysis
4. Pre-optimization of internal representation.
5. Code generation
6. Post optimization.

Can some one please explain about 4, 5 and 6 steps ? What they signify ?