Full documentation and software are available for download. It's for iMacs released in February or July 2001, which were announced at the Macworld Expos in San Francisco and New York.
The update is for those who switch off their iMac's electrical power in order to conserve energy. When the iMac is cut off from power in this way, it relies on its internal PRAM battery in order to remember the date, time and some Control Panel settings. Once the update is installed, the iMac will maintain these settings for a longer period of time when disconnected from AC power.
Comprehensive installation instructions are available on Apple's site.