It is often used in medical and biological terms to describe something that is located inside or within something else. Here are some examples:
* Endocardium: The inner lining of the heart.
* Endocytosis: The process by which cells take in substances from their surroundings.
* Endocrine: Relating to glands that secrete hormones directly into the bloodstream.
* Endometrium: The inner lining of the uterus.
* Endoplasmic reticulum: A network of membranes within the cytoplasm of eukaryotic cells.
So, when you see the suffix "endo-" in a word, you can generally assume that it refers to something that is located inside or within a structure or system.