If you have a compatible MacBook then the feature will stop your laptop from overcharging if it detects that you leave it plugged in most of the time, and won’t continue to charge the battery at 100%.
Lithium ion batteries degrade over time significantly worse if kept charging at 100% constantly. This isn’t usually an issue for smartphones, but many people use a laptop at a desk and may keep it plugged in all day and night. Apple has acted with a software update that is on by default (you can turn it off if you want) that will learn from your use and keep the battery hovering below 100% with clever background charging throttling.
A similar feature is on iOS 13 where your phone will learn to slow charge overnight if you always plug it in and then take it off charge during a certain window of time. The iPhone only charges to full capacity shortly before your normal waking time.
Aside from this, Catalina’s latest update also gives you the option to turn off video window resizing during FaceTime group calls as well as a host of bug fixes and improvements to system performance that you’d expect to find in such a point update.