The ileocecal junction is a sphincter or valve connecting the end of the small intesting (the ileum) with the beginning of the large intestine (the cecum). If you run your fingers along the small intestine until you reach a bump and the small intestine appears to turn into a much thicker object, you have reached this junction. It is much larger and more pronounced in herbivores.

In this picture the probe points right to the junction.