This is not a connection for video, but rather is the slot for Kensington projector locks. Projector locks are only used if you suspect or anticipate the projector will be installed in a high theft prone area.
The one end of the lock goes into the Kensington slot on the Optoma projector; while the other end of the Kensington cable can be used to loop around a stable structure like a projector pole or mount.
Locks typically protect the projector from being stolen if installed properly; as only the person with the key can open the lock.
Figure 1.1 Optoma Projector
Figure 1.2 Standard Kensington Lock cable with its respective key
Figure 1.3 Example of how a lock cable is used to secure a projector with its pole mount
Note: Images above are just example purposes only, as your specific cable or projector model may differ.