Quoting directly from the UCSC website:

"ChrUn contains clone contigs that cannot be confidently placed on a specific chromosome. For the chrN_random and chrUn_random files, we essentially just concatenate together all the contigs into short pseudo-chromosomes. The coordinates of these are fairly arbitrary, although the relative positions of the coordinates are good within a contig."

https://genome.ucsc.edu/FAQ/FAQdownloads.html