This means that the Subversion repository on svn.typo3.org is no longer updated and will be removed at some point in the future.
Git is a Distributed Version Control System, which offers more features than the centralized Subversion. However, it's not the variety of new features offered by Git that were the main reasons for migrating. Most important was the availability of a web-based review system called Gerrit, available at review.typo3.org.
Gerrit now takes the place of the Core Team mailing list, which had been used for peer-reviewing the changes (RFCs), before committing them to the official repository. This mailing list will remain available to discuss Change Requests outside of Gerrit.
The TYPO3 Core Team is aware of the challenges introduced by Git and Gerrit, not only for the team members themselves, but also for the many contributors. The new workflows are more or less settled and we have created some Contribution Walkthrough Tutorials to guide you through the required steps to review or submit patches using Git and Gerrit.
To make it even easier for contributors take their first steps, the next Bugday on Friday, March 25th has been declared a "Git day". Several developers will be available in IRC to help out when problems arise while pushing new changes to Gerrit or pushing old, still pending RFCs from the mailing list.
Extensions using the Subversion server are not affected by this migration. They will stay with SVN infrastructure. Plans for a migration to Git (optional or mandatory) will be made within the next few weeks.