Extensions allow certain apps to interact with one another. Not every app is compatible, but extensions can be a great timesaver once you get used to them.
Let’s take photo editing as an example. The Photos app has some decent tools, but what if you want to use the app in conjunction with another photo editor?
Open Photos and select a picture. Tap Edit followed the … icon and then More. From the next screen you can enable any compatible apps for use as an extension. Once enabled, these app icons will appear next to the More button on the previous screen.
Tap one of the apps from this menu to open a limited version of it within the Photos app. Most of the extensions available provide additional filters or editing functions. You can do whatever you like with the bespoke features and then tap done to return to Photos.
Likewise, there are plenty of useful extensions available for other apps – Safari in particular has some great ones.