Rebalance is the process of redistributing shards evenly across all eligible (non-excluded) nodes after a cluster has been resized. In most cases you expand a cluster, adding nodes so the cluster capacity matches the growth of your data. However, you can also choose to reduce the number of nodes in a cluster.
You do not have to rebalance the import or API nodes after resizing the cluster. You just have to restart the tier after resizing so all the nodes in the tier are used correctly.