What does the following code do?

def mystery(a_list, b_list):
    my_stack = Stack(len(a_list)+len(b_list))
    my_list = List(len(a_list)+len(b_list))
    
    for item in a_list:
        my_stack.push(item)
    
    for item in b_list:
        my_stack.push(item)
    while not my_stack.is_empty():
        my_list.append(my_stack.pop())
    return my_list