What does chrUn mean in the output from a Bowtie run on human sequences?
I presume chrUn_KI270424v1 refers to a scaffold or sequence that has not yet been assigned to a chromosome. The designation after the Un (unknown or unassigned?) may refer to a particular single sequence or a scaffold. You will be able to determin this by examining the reference sequence and its documentation.

(My assumption is based on the sort of thing one finds in the various versions of Drosophila reference genomes, with which I am familiar. So I can’t help you with the specifics of the human genome.)