This warning sign, which usually appears as ``'', indicates that the link it marks takes you into ACL2's online documentation.
The documentation is a vast graph of documented topics intended to help the
user of ACL2 rather than the potential user. If you are exploring
ACL2's home page to learn about the system, perhaps you should go back rather
than follow the link marked with this sign. But you are welcome to explore
the online documentation as well. Good luck.