My dentist says i need a filling by the gum line of a crown, it just occurred to me i have never heard of this before.

Not surprised..: .. there are a lot of things that patients don't know, and that's why we're here to help. Your dentist is trying to save you the expense of fabricating a new crown now that the margin of your old crown has decay. The crowns don't last forever, so this is a way to get a few more years of life out of it before you need to replace it.

But: But your Dentist did, often. Trust him/her.