It's not a mode that would be used often as it's purpose is to protect the battery if the Steam Deck is going to be inactive for a long time. Most people will only encounter it when they turn on their Deck the first time, as Valve ships them in that mode. You have to go into the BIOS to activate it, and when you want to use the Deck again, it will only turn on by plugging a charging cable into it.

iFixit has a step-by-step guide: https://www.ifixit.com/Guide/How+to+Enable+Steam+Deck+Battery+Storage+Mode/149962