Something that is supererogatory is a little too much — more than what you want or need. It could be an extra layer of tinsel on an already flashy Christmas tree or a second lengthy apology after the first one was accepted.
You could probably argue that the word supererogatory might be, well, supererogatory. After all, the word superfluous is very close in meaning. And you might notice that excessive is a shorter, more common synonym for both of those words. Necessary or not, if you encounter supererogatory just remember that it means "too much," even to the point of creating a slightly bad impression.