This tab provides a screensaver-style interface to the APM suspend/standby modes. It allows you to configure different wait times and/or actions depending on whether or not your laptop's plugged into the wall. It's intended to be used in addition to the existing KDE screensavers - you could for example disable suspend/standby when plugged in to the wall, but enable suspend with a shorter time than your normal screensaver kicks in while running on batteries.
'Standby' mode seems to do different things on different laptops - in particular on some it may not stay in standby mode and your laptop may be continually waking/sleeping if you leave it in this mode.