What is a patch?

A patch is a structured file that consists of a list of differences between one set of files and another. All code changes, additions, or deletions to Drupal core and contributed modules/themes between developers are done through patches.

Patches make development easier, because instead of supplying a replacement file, possibly consisting of thousands of lines of code, the patch includes only the exact changes that were made. In effect, a patch is a list of all the changes made to a file, which can be used to re-create those changes on another copy of that file.