It forms part of the maintenance step in a software development process, and it is commonly used for fixing security issues in older versions of the software and also for providing new features to older versions.
The simplest and probably most common situation of backporting is a fixed security hole in a newer version of a piece of software.
[1] In real-life situations, the modifications that a single aspect of the software has undergone may be simple (only a few lines of code have changed) up to heavy and massive (many modifications spread across multiple files of the code).
In the latter case, backporting may become tedious and inefficient and should only be undertaken if the older version of the software is really needed in favour of the newer (if, for example, the newer version still suffers stability problems that prevent its use in mission-critical situations).
[2] The process of backporting can be roughly divided into these steps:[1] Usually, multiple such modifications are bundled in a patch set.