I'll explain all of these points. These statements are often named LAP, for Lisp Assembly Program. With the proper documentation, you can write LAP code and have it invoked by your own functions. You need to know how to use reserved registers, what subroutines to call, what stack protocol to follow, and many other low-level details. Some Lisp systems are better than others at including explanatory comments with the disassembled code.
Declarations are forms that provide advice to the compiler.
Here, I've said that speed is of ultimate importance, and that I could care less about runtime safety or debuggability. If the compiler pays attention to all of this, I should get code that is optimized for fixnums, runs fast, and falls over if I pass it anything other than a fixnum or cause it to generate a result that isn't a fixnum. Looking at the generated code, it appears that the compiler has paid attention to my declarations. The addition is performed by PowerPC instructions instead of a subroutine. In fact, most of the code in INT-ADD1 has to do with entering and leaving the function.