an iodine dip is when you get a concentrated solution of iodine and add it to a small container with tank water, and then dip your coral in it for about 1-2 minutes in order to kill any hitchiking parasites like bristleworms, flatworms, aiptasias, etc.
most of the time, you cant see parasites until they multiply in your tank, so an iodine dip is pretty essential whenever you buy coral, regardless of the source.
but like i said... dont do it to a clam