Why is the word FISH on the back of some trucks?

It is required by law for trucks carrying fish in interstate commerce (unless more complex markings are used) as a shipping/import/export control. The statute is at 50 CFR 14.81 and 50 CFR 14.82.

The above is accurate, however the true reason is because fish contains the highest levels of Iodine than any other food, in a normal diet. Many people are highly allergic to Iodine and if they come in contact with Iodine they could possibly go into anaphylactic shock. It is to notify all personnel involved in the shipping/transporting of fish, such as dock or warehouse workers that a particular truck contains Iodine (FISH).