This is the 3147th most frequent Korean word.
거는
"거는" means "that is" or "the thing that is" depending on context.
Here, '거는' is used as a conjoining form derived from '것은', meaning 'the thing that is' or 'as for this matter'.
나는 그것이 중요한 거는 알아.
I know that the thing (fact) is important.
In this case, '거는' comes from the verb '걸다' (to initiate a call) used as a modifying adjective form.
네가 거는 전화는 언제나 반가워.
The calls you initiate are always welcome.
Here, '거는' also comes from '걸다' (to hang or place), showing the expectations being directed towards something.