Second person universal is a fancy term for what some might think of as semantic sloppiness. It is a way of using the second person mode of address in a theoretical or universal sense.
Since in English, the second person pronoun is you, this would result in statements like "You should use a dash of salt when you cook rice." In this statement, the "you" doesn't refer to any specific person, not even the person being addressed. It simply means that anyone who wishes to cook rice should use some salt.
The use of the second person universal can cause some misunderstanding, for example, if I said "You know when you get high on cocaine, you get all sketchy and you get nyastygmas", a person who was addressed by this may think I was insinuating they were a cocaine user, when actually I am just stating that most people who use cocaine start acting weird.
In proper English, the second person universal is One, as in "One should always eat their (some eight years after writing this, hapax (like Kether and cheese, or something), messaged me to let me know that I should be really proper and say "one's" and not "their", which is true, and after 13 months, I actually corrected for it)crumpets before playing croquet", but I personally always feel so stuffy when using the word "one" in this context.
The Rastafarians use I&I for both the first person and the second person universal, so the sentence about cooking rice would come out totally different, even disregarding the fact that Rastafarians would never add salt to their rice.