  Thanks to the recent advances in metamaterials, reconfigurable intelligent
surface (RIS) has emerged as a promising technology for future 6G wireless
communications. Benefiting from its high array gain, low cost, and low power
consumption, RISs are expected to greatly enlarge signal coverage, improve
system capacity, and increase energy efficiency. In this article, we
systematically overview the emerging RIS technology with the focus on its key
basics, nine fundamental issues, and one critical problem. Specifically, we
first explain the RIS basics, including its working principles, hardware
structures, and potential benefits for communications. Based on these basics,
nine fundamental issues of RISs, such as ``What's the differences between RISs
and massive MIMO?'' and ``Is RIS really intelligent?'', are explicitly
addressed to elaborate its technical features, distinguish it from existing
technologies, and clarify some misunderstandings in the literature. Then, one
critical problem of RISs is revealed that, due to the ``multiplicative fading''
effect, existing passive RISs can hardly achieve visible performance gains in
many communication scenarios with strong direct links. To address this critical
problem, a potential solution called active RISs is introduced, and its
effectiveness is demonstrated by numerical simulations.
