A guard page is a special page in memory that has its OS-level page protection bit set. Thus, trying to dereference the page will throw an exception. This is used, for example, as a mechanism to detect stack overflows by keeping guard pages at the end of the stack. It can also be used for implementing safepoints, by protecting a previously unprotected guard page that is dereferenced from the generated code at a given safepoint. This makes the runtime throw an exception the next time that safepoint is reached, and control can be halted in a structured manner.
See also Safepoint and Livemap.