Hello, and welcome to the 5.6.0 and 5.5.3 release of Kibana!
The 5.6.0 release is an important one because it contains the features to allow upgrading to 6.0 including backward and forward compatibility and the Upgrade Assistant. We didn't include any significant new features in 5.6.0 as our focus was on upgrading and compatibility.
Upgrade support in 5.6.0
As part of X-Pack Basic (free), we are providing an upgrade assistant to aid in migrating to version 6 of the Elastic stack. The upgrade assistant identifies breaking changes to index and cluster settings and allows you to enable deprecation logging. Internal indices used by the stack will need to be reindexed, in addition to any indices created in 2.x. The assistant provides a one-click solution to these migrations.
After resolving deprecation issues and migrating indices, you are ready to upgrade your stack. Elasticsearch 5.6 supports rolling upgrades to 6.x, and Kibana 5.6 can run in tandem with an Elasticsearch cluster undergoing a rolling upgrade. Once the full Elasticsearch cluster has been upgraded, you can upgrade Kibana to 6.x.
The 5.5.3 bug fix release
The 5.5.3 release contains a number of bug fixes and enhancements.