Not being familiar with which engine/tranny combination you have, I will offer a wild guess based on other towing platforms I've owned...
MOST trannies have a tow/haul mode which is used by some to change the tranny programming a bit to hold a gear a bit longer, and some downshift sooner in the expectation that you are hauling a LOT of weight...
99% of the time I never use it - although the engine braking when it downshifts helps you when stopping...
If the light is on it either means you/or someone pushed the tow button
or less likely there is a problem with the tranny (but typically it blinks if there is a problem and the tranny acts up)...
this is a bad picture I found on the net - but it LOOKS like the stalk has a push button on the end ?!? click on the pic for a closer look...
look on your shift stalk for a switch/button etc... that you can push to toggle tow mode off... (it may be on the end of the stalk)
ah, and on edit, the pic tells me you have a ford chassis, so those are what I have a bit of experience with (having owned about 10 or 20 fords over the years
)