How do I clear closed programs from memory, Samsung Galaxy S4?

[AC Question]

How do I clear closed programs from memory? Samsung Galaxy S4

Ive done it accidentally three times but cant figure out how to do it. I close apps Ive been using but they arent really closed, they are stacked in the memory.
How do I clear them out?

 

[extraclass]

Hold down the HOME KEY a couple seconds and a screen will appear with all of your OPEN Apps. Press the button at the bottom right CLOSE ALL.

I have found that closing these apps saves my battery.