You often hear how Lisp lets you define your own domain-specific language via macros. Well this one is not the case.
Instead, it is an elaborate virtual machine with own semantics/instruction set. From these, one can construct complex operations over data that are then composed into single program which can be compiled at run time (Common Lisp has compiler accessible as a call in “standard library”). As most Common Lisp implementations compile to native code, this is indeed a domain-specific JIT compiler as advertised.