The ileocaecal valve is composed of two mucosal folds and located at the connection of terminal ileum and caecum. It prevents reflux of intestinal contents from the large intestine, which is strongly colonised with bacteria, to the small intestine, which is only sparsely colonised with bacteria.
The ileocaecal valve is an isthmus and as such a predilektion site for swallowed foreign objects to get stuck. The valve can be stenosed when the patient suffers from an inflammatory bowel disease (e.g. Morbus Crohn).
(image source: gastrolab.net)