It used to be a toggle in the options in previous (VERY much older, though) models but it's been a while since Samsung just put the 'Sunlight' mode as the default for Auto-Brightness. Same thing with 'Glove sensitivity' mode...now the screen sensitivity is enough to be used with gloves, but not as strong as before, so thicker gloves won't work.
Anyway, sunlight mode just boosts brightness and color display a bit so the screen is more usable when on full-on sunlight. You can, of course, always cover the light sensor on the front and the sunlight mode will be turned off.