Artificial General Intelligence is roughly defined as a machine that can do all the things a human can do. For example, an AI capable of doing AI research and programming computers, would be AGI. The Turing test (as Turing originally described it, not garbage like chatbot competitions) is the most widely accepted standard of AGI.
Superintelligence goes well beyond that. A machine with cognitive abilities far beyond humans. I don't think such a machine is very unlikely even in the near future. It's unreasonable to believe that humans are the pinnacle of intelligence. We are just the very first intelligent creature to evolve. Our brains are heavily resource constrained by size and energy. And neurons are many orders of magnitude slower than transistors. And also far larger and less compact.
Superintelligence goes well beyond that. A machine with cognitive abilities far beyond humans. I don't think such a machine is very unlikely even in the near future. It's unreasonable to believe that humans are the pinnacle of intelligence. We are just the very first intelligent creature to evolve. Our brains are heavily resource constrained by size and energy. And neurons are many orders of magnitude slower than transistors. And also far larger and less compact.