Some features need to utilize exec commands, for example to convert a non-mp4 format video into a web optimized and streamable format.
You can also analyse the source code and search for those references and see which functions they are used in. This will give you a good understanding where and what they are used for.
In case Clooud tells you that these functions are not enabled, don't worry. You can still use all of Clooud! It is just recommended that these functions are enabled, because it enables some features, like automatically generating thumbnails for videos for example.
Now, how can I enable it? Assuming you use a web host like Godaddy or BlueHost, you would have to contact their support. In case you use your own server and for some odd reason these functions are disabled, you will have to change you php.ini's "disable_functions" string. Just remove whatever function you need out of it, and it will work.