'Uththara Kuru' means lands to the north of Kuru regions. The northern-most regions of ancient India extend upto the borders of present Kajakisthaan etc. Those lands were considerd very inaccessible those days. Conquering them was a mark of achievement.
While Arjuna went on campaign to conquer the northern side provinces, he reached that far. The inhabitants were wondered at his intrusion and presented him with good gifts for the Raajasooyam.
At many places, the Uththarakuru lands were mentioned. For exmple, sage Agashtha said to the other sages taking part in his sacrifice that he could even get the things available in Uththarakuru lands for the sacrifice if need be.
"After praying to Naaraayana, Nara, Sarasvathi and Vyaasa, then one should read the Jayam."
DEDICATED TO MY DEAR NIECE VYSHNAVI KRISHNA