Yep— there have been a lot of changes both with the industry as a whole and with smartthings specifically in the last year. It’s been hard for any of us to keep up! 
As far as matter, the short answer is that it’s a new industry standard which is intended to make it easier for Home Automation Devices to work together. The idea is that a Home Automation device manufacturer would only have to create one integration, and at least some features of the device would then work with SmartThings, Apple Home, Google Home, Alexa, etc.
It’s a great idea in theory, but the rollout has been pretty bumpy in practice, so it’s still something that has yet to achieve its potential. But all of the big players are on board, so it does seem to be coming.
Consumer reports has a particularly good explainer article here:
Matter Smart Home Standard FAQ - Consumer Reports
And here’s the discussion thread from this forum about the smartthings – specific aspects of matter:
Matter - smart home connectivity standard (formerly Project CHIP)
SmartThings hubs have been updated so that they can bring matter certified devices into your smartthings account. However, Samsung has not yet made the commitment to have smartthings hubs be “matter bridges” so you can’t add a smartthings hub to other matter – compliant apps (like Apple Home or Tuya Smart Life). Maybe someday, but for now, the smartthings/matter integration is one way in. Not out.