The apps listed in ATK are "open" to an extent. Let's use an analogy...
Lets say you're sitting at your desk and you are working on several projects at the same time. You take the projects folder out of your file cabinet and open it on your desk. If you are writing an email, and need to reference one of your projects, you need only look at the open folder sitting on your desk, rather than having to walk over to the file cabinet, retrieve the folder and open it. Thus, saving you time. At some point, your desktop will fill up. So, in order to open an additional project folder, you would need to close and remove one that is already there, to make room.
Android works like this. The applications in the ATK list are like those projects that are laying open on your desk and the space on your desktop is your phone's RAM. It can keep several programs, "ready to go," in case you need them without affecting performance on your phone. And...if it needs the room, it will select a program to close so that everything runs smooth.
Make sense?
There is usually nothing wrong with letting ATK close programs, but in some cases, ti will not allow the programs to run like they should. Most programs that have tasks to do based on a schedule you set, will tell you to exclude them from the "kill" list in apps such as ATK. It's trial and error for some.