Let's begin with an example that might have some familiarity if you or more likely one of your parents has had a recent physical exam.
It is common to get handed an envelope on your way out  containing a collection device with the instructions to put a stool sample in it and send it back to the lab listed on the envelope.
This sample will be used to carry out a Fecal Occult Blood Test (an FOBT) to check for the possibility of Colorectal Cancer.
"Occult" refers to "hidden" blood that might be present in the sample, and that could reflect the presence of colon cancer.