Prior to this date, new versions of packages will be automatically imported from Debian where they have not been customized for Ubuntu, that is when the version number of the package in the current Ubuntu development branch does not contain the substring "ubuntu" and there is a newer version in Debian. Imports from Debian are from the unstable branch. (A historic note, for some LTS release 12.04 and lower were done from testing, but since the introduction of ProposedMigration syncs happen from unstable all the time.)
Entirely new packages (those not in Ubuntu at all) are also automatically imported prior to this date. The import is done by copying the source package verbatim from Debian and building fresh binary packages on the Ubuntu autobuilders.
By this date, all packages that have been modified by both Debian and Ubuntu since the last merge should have been merged at least once in the current release cycle.
After this date, packages will only be imported from Debian in this way by explicit request from a developer. (If the package needs to be modified for Ubuntu, or is not in Debian, then a developer can of course also upload it directly.)
To implement this freeze, somebody with access to the ubuntu-archive user on snakefruit.canonical.com should comment out the auto-sync entry in its crontab.