Daylight Left works out today's sunrise and sunset for your location from your coordinates and the date, then counts down the daylight that remains. Before sunrise it counts down to first light instead.
The maths runs on your device using standard solar position formulas, so no location data is sent anywhere. Times are shown in the local time zone of the place you picked, not your device's.
+Is daylight left the same as the time until sunset?
Almost. Sunset is a single clock time; daylight left is the distance between now and that time, which is what you actually need when you're deciding whether to head out.
+Why does the amount of daylight change every day?
The Earth's axis is tilted, so as it orbits the Sun the length of the day shifts. Near the equator the change is a few minutes across a year; far north or south it can be several minutes per day.
+Does it work in the far north or south?
Yes. During polar day it says the sun doesn't set today, and during polar night it says there is no sunrise, rather than showing a nonsense countdown.