Debian releases a new stable version approximately every two years. During its life time, a stable release usually gets security updates only, but in general no feature updates.
For some packages it is desirable to get feature updates earlier than with the next stable release. Some new packages included in Debian after the initial release of a stable distribution are desirable for stable too.
Both use-cases can be solved by recompiling the newer version of a package from testing/unstable on stable (aka backporting). Packages are backported together with only the minimal amount of required build-depends or depends not already fulfilled in stable (if any), and without any changes unless required to fix building on stable (if needed).
There are official Debian Backports available, as well as several well-known unofficial backports repositories.
Debian Fast Forward is an alternative Debian repository containing complementary backports from testing/unstable to stable with packages organized in a curated, self-contained selection of coherent sets.
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