When saying what are the things to know in CS, I assume you mean “you” as in a collective you — i.e. what does the field know?
We know a lot of things, but the most important and most fundamental point of knowledge is that computers have hard limits on what they can do.
This is due to the Theory of computation.
Among other things, the theory of computation says that any solution requiring infinite resources (time, memory, input/output, etc.) is not possible to solve on a computer.
We also know all solutions (algorithms) take non-zero resources, thus the idea that “information is free” is a misconception.