1st answer: its the Endurance Rating based on GSMArena's testing profile. It's basically how many hours you can use it with 1 hour of 3G calls, 1 hour of video playback, 1 hour of web browsing daily, then set on standby. You can learn more here:
However, this entire article is wrong/flawed because GSMArena's testing, especially before 2016, is flawed because of their differential screen brightness level, video media used during playback, differences in software, and the introduction of LTE into the market. Essentially a current phone (2018) might have a similar figure to a phone much older (2013), but the older phone was tested using a dimmer screen, lower quality/less taxing video, and slower Data Connection. Or quite the opposite, a new phone (2017) might have drastically higher Endurance Rating figure than an older phone (2014), and the older phone may have used the same Data Connection and Taxing Video Playback, but the older phone may have been tested at a much higher brightness rating (eg 310 nits) as opposed to the new device being tested at the standard 200 nits level. We will never know which scenarios come into play on certain handsets, the only solution would be to source Hundreds of phones, and refurbish them with Fresh Batteries, and run a properly controlled Scientific experiment on all of them.
The sad part is that the efficiency doesnÂ’t grow, in part due to technology upgrades being used to make chips faster instead, and due to displays being bigger and brighter.