The WebAssembly spec states here that it has an implicit operand and call stack.
What exactly does that mean in terms of WebAssembly, and how would and explicit stack differ from an implicit one?
The implicit stack is managed by the VM and is not directly accessible. It is implicitly pushed to, and popped from, by various instruction.
An example of an explicit stack might be a region of linear memory that you have direct access to via load and store instruction. Indeed this is exactly what llvm does for address taken stack variables. i.e. it allocates a specific region and linear memory for them.
The control flow stack (e.g. the return addresses of each of the function on the stack) is also part of the implicit stack and cannot be explicitly read from or written to.