The lever you are referring to has 3 positions:
1) Locked Towing-This is when the hitch is positioned all the way forward for towing. The lever will lock the hitch to the slide rail.
2) Unlocked-Place the lever in this position when you want to slide the hitch forward or to the rear. DO NOT tow in this position.
3) Locked Maneuvering-This is when the hitch is positioned to the rear of the slide, allowing you to tow safely when backing into a tight spot or turning sharply. This allows for additional clearance between the trailer and the cab of the truck.
When sliding the hitch on the rails, make sure it is positioned completely to the rear or completely to the front so the arm can lock it in place.
If the slider is not used often, it can get rusted and this can make it really difficult to get the hitch to slide. When this happens, lubricate the slide and rollers with WD-40 or a rust penatrent. I have then seen people place the lever in the UNLOCK position and with the trailer connected, drive forward or in reverse, making sure that you manually apply the trailer brakes using the brake controller. This will move the hitch into the desired position. They only move the vehicle a few inches to get the hitch to slide while preventing the trailer from moving. I always use wheel chocks on my trailer when hitching or unhitching to reduce the movement of the trailer.
Keep the slider lubricated with WD-40 to prevent it from rusting.
Good luck and be careful to make sure your trailer is properly hitched.
Here's a good video for you to watch:
Hitching a fifth wheel trailer by RV Education 101® - YouTube