Lower courts follow (or are supposed to follow) decisions of higher courts in some, but not all circumstances.
For example, say the Court of Appeals throws out a felony conviction and the charge was made because someone that legally built a pistol and filled out the RI-60 as both purchase and seller.
And let's say, hypothetically, that the appeals court held that the law was unconstitutionally vague or overbroad because it jeopardized both people fraudulently registering illegal pistols along with honest citizens trying to get their lawfully built guns registered, the lower courts (circuit courts and district courts) would have to treat other cases the same way. That would be "case law" - a higher court would have created a legal precedent from that case that would apply to similar cases.
I am not suggesting that an appellate court would rule this way. Just hypothetical.