Please explain

as far as I know, the Jewish religion forbids them to eat a lot of foods, including seafood ( fish is allowed, though). I'm not sure which is the explanation, but it can be found in their holy books . it has something to do with the idea of seafood being "impure"...

As someone has all ready pointed out, that joke was in reference to Jewish dietary laws. In the Old Testament the Jewish nation was instructed by God not to eat certain foods. This included pork of any kind, any animal with a cloven foot, shell fish as well as many other types of sea animals, and many others. Today, Orthodox Jews still follow these strict dietary rules although Conservative and Reformed Jews are not necessarily as strict. (For example, Jews cannot mix dairy and meat products even going so far as not being able to prepare the dishes in the same utensils. So an Orthodox Jew could not have a cheeseburger even though they aren't forbidden from eating either cheese or beef. But eating the two at the same time is forbidden. Conservative and Reformed Jews may not be so strict in this but Orthodox Jews do follow the requirements) Hope this helps some.