It's a manual shift function. I have the Tow/Haul and compression brake features on my 2017 so I don't generally use the manual function as much as I did with my 2014 6.4L HEMI. I used it all the time in that truck.
Operation is described in your owners manual but basically, once you put the vehicle in drive, you can push the - button and it will shift down a gear. If you are stopped, it will go to 1 and then hold that gear. It won't hold it forever and will shift to protect the engine at a pre-determined RPM. As you move faster, you can shift up at your discretion using the + button.
When shifting down, make sure you don't shift down from too high a speed and over rev the engine. Also, don't forget to shift up when appropriate.
With a little bit of practice, keeping an eye out on RPM and gear settings, you'll get a feel for when to shift up or down without having to look.
I like the manual mode function. It's great for holding the truck in a particular gear around town, gives you better throttle response at low speeds because you aren't lugging the engine and can be used on freeway entrance ramps to pick up some speed by kicking down the transmission for a bit more power.
Take a look at your Owner's Manual.
Or, it simply limits the truck from shifting to any gear higher than you set.
This is truly what it was intended for. It was never meant to turn an auto trans into a "manual", although many, including me, do use it like that. Every day. Along with turning off traction control and turning on tow mode.
One bit of warning...
If you're shifting using +/- and also have cruise control set, you will inevitably, one day, slow or speed up the cruise ctrl to stay with changing traffic speeds using the + and - steering wheel buttons. HOWEVER, every once in a while, you won't use the steering wheel buttons, you'll use the stalk +/- to "slow down 3 or 4 mph". EXCEPT, you've just DOWN SHIFTED 3 or 4 gears and pinned your tach. Your Ram is quite obedient, so when you go from 6th gear to 2nd gear, your butt will pucker up. Your brain is sluggish and the truck will frickin' hold 2nd gear like you told it to. Especially if in Tow mode! You begin yo change lanes to look for a run off area. Everybody in the truck is like, Dude! WTF?
THEN you figure it out and push + + + + + but, for some still unknown reason, these trucks like to STAY IN 2ND GEAR!. Sometimes for 6 or 8 or 20 seconds before upshifting.
Now, everybody in the truck thinks you're a giant ******* and that you did it to pin them in their seat belts on purpose. You explain it and your wife tells you to just leave it alone. The truck is smarter than you, obviously! Jerk!
You appologize, you feel small and consider bringing it in to have it checked but you know they won't find anything wrong. So you drive it like you used to and just ignore all the plusses and minuses on everything
The End.
Fess up fellers. I've done it 4 or 5 times in 2 years. Hahaha.
Any other honest Abe's? Come on...