What basic knowledge is required to understand SIKE?
Craig Costello has tried writing precisely what you're interested in, see SIKE for Beginners. It refers to numerous other surveys for isogeny-based crypto which may be useful (lecture notes by De Feo, and surveys by Galbraith-Vercauteren and Smith).
Galbraith also has a book on the math behind crypto. Chapter 25 deals with isogenies, so may be useful for mathematical background (but does not discuss SIKE).