The Stand-By graph uses smart learning algorithms to detect the total power usage of devices that are always on. The Stand-By graph can show you if you start using more or less power over time with dormant devices. Or can let you know if you left something was left on if it has suddenly spiked. The graph shows your regular Power Usage graph in the background, with an overlay of the detected Stand-By power level per period.
The Stand-by graph is enabled automatically if you have signed up for Energy+ at Settings > Energy+.