What in God's name is X11

A quick Google search would have resulted in several links about the X Window System. It appears that the application uses it for whatever reason (probably a cross-platform thing), but since X11 is no longer included with OS X, a separate install will be needed if you want to use that particular program.

If you install X11 the app you want to use will work. If you don't you will need to find a different app, one that does not need X11.

The X Window System is a different way of managing the windows that you see when you use your Mac. The two systems aren't completely compatible. The reason why the app you wanted is in the App Store but needs the X Window System is that X was included until (I believe) Mavericks, so it wouldn't have been can issue, and things would have worked seamlessly just like you expect.

As others have stated, installing X will fix the problem with the app. I've never done that on a Mac before, so I'll leave that explanation to those who have. But perhaps this gives a tiny bit of background for you.

X11 and OS X Mavericks

Unfortunately, computers (like cars, televisions, ping-pong paddles, and just about anything else involving technology), require a certain amount of knowlege, even if it is just to know what the words mean. The graphical user interface (the screen images and mouse stuff) is not something that is just there, it takes a lot of different people create it, so there are many different systems and versions in use.

X11 is one of those different systems, and was originally a part of the standard OS X installation, since it is used in various other UNIX-like systems, and was probably used by the developer as a way to avoid having to make several different versions of the application. Currently X11 is not included, so it would be up to the developer to either update the application or inform potential customers of its requirements.

Most applications in the App Store are not from Apple - the product page provides information about the particular application, including the seller/developer. Apple gives applications a quick test to make sure that they run and follow the App Store guidelines when they are first submitted, but the developer would be responsible for keeping up with new OS releases.

I suggest you delete that app. It doesn't look right. What are you trying to accomplish?

Just so you know, X11 isn't some sort of random spyware or anything like that. It is a framework that people use when developing applications. Just like you can run a car off gas, fuel, electricity, you can use the X11 framework or a different framework when you develop an application. You might get a little bit better idea from http://www.imore.com/x11-and-disturbing-trend-apple-removing-functionality-os-x

Have a look at Handbrake (http://handbrake.fr/) and see how that goes for you.

I'm not knowledgable about video editing, and even less so about Adobe products, but I do know that you should not convert video to a lossy format before editing it. That's something you do after editing. I doubt that any free app from the App Store is going to help you. A solution to your problem is more likely to be found on the Adobe website.