It may be helpful to read the wikipedia article first:
Seems more than I are having trouble understanding what multimethods are, as with most things i suspect there is a simple answer buried under all the lingo...
After reading the wikipedia it seems multimethods are not owned by a class and and are dispatched when the arguments of the method match exactly on runtime type or argument pattern?
i don't have experence with them yet i am starting to get to the OOP side of nim.
You can think of multiple dispatch as a version of overloading that is resolved at runtime (rather than at compile time).
That said, actual multiple dispatch is mostly something that has a theoretical appeal, but sees little applicability in practice and has a number of downsides (difficulty to implement it efficiently, adverse effects on modularity, difficult to predict behavior based on unintuitive specialization rules, etc.).
In most practical situations, you'll be fine with using single dispatch.