The words written earlier in this thread well-describe the latch rod/pin, but a picture is worth a thousand words. If any of the responders is at their boat, a quick picture may help. I’m 750 miles from my boat or I would post a photo.
I have had the rod/pin on my TV mount jam a few times. If that is happening in this case it would be most confusing and frustrating. The only advice I can offer if the rod/pin seems to be jammed is to lubricate the rod/pin and gently rock the bracket while pushing on the rod/pin until it frees up. To help identify the rod/pin, it transits through a spring that returns the rod/pin to the “lock” position when the TV is finished moving to the UP or DOWN position. There, more words. A picture would give them context.
MSU1966, if you can determine the maker of the bracket, a manual for it may be found online.