What does it mean exactly? Or what is novel here?
That requires a linear search through a list of all possible combinations of input arguments.
In a single dispatch language like most object oriented languages, you can do a simple dictionary/hash table lookup. Much faster.
With the JIT Julia is able the optimize away most of these super slow lookups at runtime. Hence you get multiple dispatch for all functions but with fantastic performance. Nobody had done that before.
It also has a feature (I can’t recall the name) which is a bit like fexprs (let’s say macros who’s inputs are the types of the arguments of a function) that can generate customised code (eg an FFT depending on the input size) on the fly.
(but I don't find it helpful to compare to fexprs, which I think of as more about deferring evaluation, whereas generated functions are about "staged programming".)