What is this icon at the top left corner?

It is the Sunrise/Sunset complication (shown in the "Utility" face).

As you know you can press gently on the watch face and bring up the customize screen. Then swipe from right to left and you will see "hotspots" where you can change the sunrise/sunset time to another piece of information that you would like better like temperature or time in another city etc

Customize your watch face - Apple Support