If you're talking about the light on the front of the trailer on the street side by the A-frame (battery/gas bottle/jack) area, it's to help you hook up/unhook in the dark. There should be a switch built into the bottom of the light. Run your finger under the light and you'll find it.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10