This is the 20th most frequent Hebrew word.
כבר
Already.
Here, "כבר" (kvar) indicates that the action of having lunch has been completed earlier.
כבר אכלתי ארוחת צהריים.
I have already had lunch.
In this context, "כבר" implies that the act of returning home has occurred prior to the current moment.
הוא כבר חזר הביתה.
He has already returned home.
Here, "כבר" is used to ask if the understanding of the answer has been achieved yet.