Extensions are basically libraries (.dll files on Windows and .dylib files on macOS) independently developed that Gig Performer can load so as to provide additional functionality, i.e., additional support for a hardware device, some new GUI functionality or even additional GPScript functions. For the first, an extension would make your hardware device usable directly from within Gig Performer as if it was already built-in.
For example, if you had a keyboard with a display and the keyboard's manufacturer created an extension for Gig Performer (with our SDK), Gig Performer would be able to communicate directly with your keyboard, and show various parameters, such as Song name, Rackspace name or Tempo on the display (or assign buttons and knobs with functions in Gig Performer).
Additionally, extensions may also provide panels and menu items in Gig Performer that may help to use them better.
Gig Performer 4 exposes an API (application programming interface) that allows third-party developers to create libraries that can be used to interface with Gig Performer. The primary purpose of the API is to enable the creation of libraries to manage control surfaces and other hardware that one might want to use with Gig Performer but it is also possible to create 3rd party GUI tools as well.
Extensions are developed using Gig Performer's SDK (software development kit) that is freely available under the GPL-3.0 license and can be downloaded from GitHub. Visit that link to learn more about SDK and fundamentals.