Compound knowledge is the knowledge that is based on other knowledge.
You can’t learn accounting until you learn math.
You can’t learn to improvise on an instrument until you learn the basics.
You can’t learn the nuances until you learn the whole.
If you focus on building your knowledge, and on learning new things that are based on things you learned before, your knowledge will become more and more valuable.
If you learn many basic concepts, anybody can come along and learn those concepts.
If you learn concepts, and then learn concepts based on those concepts, and so on your compound knowledge will be valuable.