Clinically, WW, it is one portion of the intestine telescoping into the adjacent part and causing an obstruction. (Tube into tube). OH dear, am in a dreadful hurry. Shll come back here to clarify if someone else does not by then.

Edit: This post now, seems self explanatory. I might be restating the obvious by explaining further. FWIW, An intussusception is an acute abdominal condition wherein one length of the intestine folds in of itself and telescopes (collapses into/ gets sucked into)into the adjacent normal part. In the process, both obstruction and strangulation occur. Common in small children.