Queue implemented using stacks

I have this problem I'm trying to solve: Implement a queue with a constant number of stacks so that each queue operation takes a constant (worst-case) number of stack operations.

I don't want assistance with the answer, but i do need help understanding the last part of requirements - "so that each queue operation takes a constant (worst-case) number of stack operations".

Does this mean that, given such a queue with n elements and another with n+1 elements, the number of stack operations will be the same, when enqueueing or dequeueing either one? What does "worst-case" mean when dealing with a queue?

Also, with a question like this, is it implied that queues are the only data-structures needed and one would not need to introduce any additional data structures to reach a solution?

I believe the question is asking you to implement the queue so that every time you perform a queue operation (enqueue or dequeue) your implementation does O(1) total operations on the stacks used to implement this. For example, an implementation that performed at most 137 total pushes and pops per time that the client calls enqueue or dequeue would satisfy this requirement, but an implementation that requires, say, $n$ stack operations per enqueue would not meet this requirement.