In a nutshell, Kentico Extensions extends the UI and functionality of Kentico using JavaScript.
Note that Kentico Extensions was not developed by, nor has any affiliation with Kentico the company.
The best way to understand Kentico Extensions is via examples:
Have you done something in one environment (Production) when you meant to do it in another (Development) ✋
The Environment Bar extensions adds a colored line and label to clearly identify between different environments.
The colours also help identify different environments within the task tray.
Tired of pressing F2, typing "custom tables" and hitting enter? ✋
The shortcuts bar adds your dashboard tiles to a navigation bar. By holding down Ctrl and clicking items, you can easily open applications in a new tab.
The shortcuts bar is based on the tiles on your dashboard.
If the shortcuts bar is covering information in the header, it can easily be collapsed.
These are just two exmaples, but Kentico Extensions can do much much more