First and foremost you should have a general understanding of type inference. You also have to understand the difference between a function and a method (in Julia's terms), see:
Functions:
https://docs.julialang.org/en/v1/manual/functions/
Methods: https://docs.julialang.org/en/v1/manual/methods/
Once that's understood, multiple dispatch is simply using all of a function's arguments inferred types to choose which method should be invoked.