Hi AMinNJ !
From a quick search on a medical dictionary:
leakage of protein from dilated lacteals in intestinal villi caused by lymphatic obstruction. May result in a protein-losing enteropathy with hypoproteinemia, diarrhea, edema, ascites and weight loss."
So, it doesn't say how common it is, but basically from what I can make out it is a leaky area in the wrinkles of the gut surface that allows proteins to escape, causing lots of symptoms that we Crohnies all find very familiar. I'm guessing that it is very common and part and parcel of the diarrhoea thing, but don't go believing everything you read on the internet now !