Mayor version bumps are a quite mayor event for KiCad.
KiCad version numbers consist of 3 numbers (Currently V6.0.10)
The third number "10" is bumped around once a month, when bug fixes are released. Updating to the latest bug fix release is always recommended (but wait a week or so if your livelihood depends on it).
The middle number is for a "quite big" update that may cause incompatibilities. It's unlikely this number will be used again now KiCad moved to a yearly mayor version.
"Normal" development of KiCad finds place in the "KiCad-nightly" version, which has a two part version number, and the 2n number is always "99", So now it is V6.99. The "99" means, "it's going to become the next mayor one. Updates are literally overnight (and nearly each night) and also have a build date and git commit string somewhere in the version info.
Changes in mayor versions are quite significant. File formats change, lots of new features get added (and new bugs too). There is a curated list on the KiCad forum, which only has announcements for new features. There are no replies or other talks in that topic, and it has 84 posts. (So that is more then one change a week!)
https://forum.kicad.info/t/post-v6-new-features-and-development-news/32633/88KiCad V7 will have most (but not all!) of the features mentioned in that list.
Some mayor ones are:
* Lots of gui fixes, colors, highlighting, shadows, line types, minimum text size, selectable fonts.
* New panels (Properties and search, sticky "hierarchical sheet" browser).
* ERC and DRC enhancements. (Starved thermal spokes, copper slivers, solder mask integrity, ...)
* Loading of bitmaps in the PCB editor for reverse engineering PCB's.
* Improved support for net ties.
* http links in texts, pdf export with hyperlinks, bookmarks, Table of Contents.
* Support for Database Libraries. (I'm not sure how complete this is at the moment).
* More automation in the Interactive router ("Unroute Selected", "attempt Finish Routing", "Attempt finish selection")
* Grouping of footprints by selection from the schematic (for initial sorting on a new PCB).
* Improvements in CLI.
* IBIS models for ngSpice simulation.