Virtual nodes

The official documentation, although verbose, provides good explanation of this.

Virtual nodes, known as Vnodes, distribute data across nodes at a finer granularity than can be easily achieved if calculated tokens are used.

In the diagram above, there are 6 physical nodes. Each contains some data as well as 2 replicas from other nodes.

The bottom portion of the graphic shows a ring with vnodes. Within a cluster, virtual nodes are randomly selected and non-contiguous. The placement of a row is determined by the hash of the partition key within many smaller partition ranges belonging to each node.

When there are vnodes, and therefore data is more sliced (notice in upper diagram, Node 5's data is only in three other nodes, whereas in lower diagram, Node 5's data is split throughout every other node), repairs take longer.