As termed in the documentation of http://lucene.apache.org, the goal of autoscaling is to make SolrCloud cluster management easier by providing a way for changes to the cluster to be more automatic and more intelligent.

So in Solr 7, there are some APIs that monitor some predefined preferences and policies. If any of the rules provided in the policies are violated, Solr changes its configuration automatically as defined in the preferences. With the updated autoscaling feature, we can now have Solr spin up new replicas depending on the monitoring metrics, such as disk space.