Explain which part? The holy water? It's fairly simple, yet very complicated. Usually, "holy water" is blessed under the Catholic religion, but it can really be anything as long as said religion has a method of blessing. Now, if said religion worships demons, things get more complicated. They come in lots of shapes and sizes. I assumed the demons referenced here would be goetic demons, but in that case, the cautions get even more complicated. Long story short, most "demons" have an instinctual hatred or aversion to things blessed under the values of their opposing deity. :3 make sense? sorta? It's complicated. I'd argue not as complicated as other things (ie, patron deities) but still hard to understand if you aren't immersed in it to start with.