Basically, It remembers the previous 2 frames, then uses them to affect the data.
Honestly, you're better off trying to recreate the effect yourself by looking at that site, you'll understand the basics and be able to adapt them to your own implementation. Don't feel bad if you don't understand it right away, it's tricky.
However, if you don't know what loopindex is, or aren't experienced in "real programming" (ie, algorithm design, with traditional programming methods) then this might be a bit too complicated to understand for now, but don't let that stop you from trying.