In this I am not sure what J21 and J1 are. Like what exactly do they stand for?

 

Your image isn't showing and won't open!

 

Connectors. One is 4 millimeters, other is 2 millimeters.

 

The "J" designator generally means "Jack", while a "P" designation generally means "Plug". So the diagram for the other part of this equipment would either have a part with a "P" designation, which would mean that it is intended to plug directly into this part (possibly via a permanently mounted cable that has a plug on the end of it) or something with a "J" designation which would imply that there would be a separate cable that connects the two. The diagram for the cable would likely have a "P" designated component on both ends.

And you must always keep in mind that all of these are conventions and rules of thumb that are adhered to and ignored in varying degrees all the time.