I wish that John McCarthy was still alive, but...
From LISP 1.5 Programmer's Manual :
LISP can interpret and execute programs written in the form of S- expressions. Thus, like machine language, and unlike most other higher level languages, it can be used to generate programs for further execution.
I need more clarification about how machine language can used to generate programs and how Lisp can do it?
All that is saying is that machine code can directly write machine instructions to memory and jump to those instructions to execute them; this is the basis of many attack vectors to break into software, in fact.
The point is, when you're writing machine code, it's easy to generate machine code. But when you're writing in a compiled language like C, you can't just generate C code at run time and then execute it - unless your program includes a C compiler.
Lisp - and, these days, many other languages, especially "scripting languages" like Perl, Python, Ruby, Tcl, Javascript, and command shells - have the ability to execute code that is generated at runtime. In Lisp, since code and data have the same structure, this is usually less work than it is in the other languages, where the code to be evaluated is generally a string that has to be parsed. (Though Perl has the ability to eval a block instead of a string, which lets the compiler do the parsing ahead of time for literal code.)