what are hard faults in the resource monitor of windows 7

A hard fault is not a memory error and has nothing to do with the stability of your memory.

What this means is, a program has asked for an address and the page it resides on is no longer located in main memory as it has either been swapped to disk or has to be referenced from the original source file on disk somewhere.

This metric can be used to find performance problems. The less hard faults an application generates, the more often the information it is requesting is found in RAM and the less often it needs to access slower storage mechanisms.