To add:
This is sign language. Sign language, though some think it is simple, is intricate and complex.
The symbols being used here are tentatively thought to mean ‘journey’ ‘lock break/break in/burglar”
This was developed for use in loud forges. Remember the part where all those hammers are ringing in the very beginning? It’s loud as balls. Or when working in a mine, say shoring up the walls when you honestly feel like loud acoustics can damage the structural integrity of your mine. [Trust me, it’s just as possible as a avalanche.] Or say that you don’t know if this new mine has orcs in it, so you can’t be loud. You would use this. While there are different dialects, all dwarves know this.
Bifur is not stupid guys. Thorin would not set out to reclaim his home and add someone who he thought wasn’t smart enough to do what needed to be done just to appease the Ur Family, who aren’t even dwarves of Erebor, they came from Moria. Bifur would not be allowed to come if he was mentally handicapped to the degree that he is being portrayed in the fandom.