meta-debian-extra has additional recipes to meta-debian. These recipes have upper version than recipes in meta-debian. For now, jethro branch in meta-debian has recipe for jessie(debian 8) package. On the other hand, jethro branch in meta-deiban-extra has recipes for jessie-backports/stretch pakcages. The branch name is one-to-one correspondence between meta-debian and meta-debian-extra.
https://github.com/meta-debian/meta-debian-extra web repo
meta-jessie-backports web subdirectory