One of the major features that Apple included in iOS 8 is the ability for app-to-app communication, which is done through something called "extensions." Extensions make for a more fluid workflow by letting you access third party app features through native iOS apps. For example, you could add the "Litely" photo editing app's extension to Photos, which would let you access its editing tools directly from your iPad's Photos app, rather than opening Litely and editing from there. Alternate keyboards are another example of extensions. There are plenty of apps that are extendable to Safari as well; we'll use Pocket for this example, which is a handy app that lets you save articles, videos, etc. for later viewing. Here's how you add an extension to Safari:
1. Open your Safari browser and tap the Share/Action button, which is the box with an up arrow in the top right of your screen.
2. On the first level of options (the sharing options), scroll to the right until you get to the "More" icon. Tap it. (Note: Some extensions will be found under the "More" menu in the second row of icons, the "action" options. Pocket happens to be a sharing extension.)
3. This brings up an "Activities" menu. If you have an app that has an extension for Safari, you will see it here with a toggle next to it.
4. Toggle it on and tap "Done" in the top right.
5. Now when you open your share menu you will see an icon for the app extension you added. To use it, just tap on it. In Pocket's case, say you come across an article you want to read but don't have time to at the moment. Just use the Pocket extension to save it, then you can read it later (and on any other device you have Pocket on).
That's all there is to it.