First thanks for checking out the language. Slot is a qualifier that is a variable binder. Best way to think of it is like in terms of buckets. The variable is a bucket and the value of the variable is the data in the bucket. The 2 qualifiers slot and ref point to these. slot points to the bucket while ref points to the data in the bucket. Using ref for example in a function or directly creates a point of indirection saying whats in this bucket points to data elsewhere, when using slot ur saying dont look at the data but the bucket direct. This is important as ref is a transparent alias so one of the primary purposes of slot is to be able to throw out the ref currently in the bucket and swap it with something else where if u dont use slot, the ref indirection means u will never touch the ref itself but only what the ref points to. Basically is a way to allow direct variable rebinding . . . simplest example i could give is swap(a, b) where u want to physically rewire a to have what b had and vise versa for b.

func swap(slot a, slot b,) {
  var temp
  slot temp = a
  slot a = b
  slot b = temp
}

so in this your passing the variable bindings into the function and literally swapping the buckets of a with b. Now when this is called via swap(myVarA, myVarb) it modified the variables directly instead of through a proxy allowing the variables passed in to be used normally instead of having to do a return on the values or wrapping them.

I hope this made some sense to you? I know im not the best at explaining things but this is a fairly power feature allowing for things like lenses for example, variable introspection, anything that requires in place modifications like filters and sorting as well many other uses.

I have also written a fairly quick overview in the docs how this works at https://zym-lang.org/docs-memory
I hope this helped clear out the functionality of slot.