The basic meaning is someone (or something) who is not necessary. A superfluous employee. I've also heard it used to describe someone who has way more than they need. Hope that helps!

superfluous means too much or redundant. For example, a deck of cards has 52 cards. If someone would like to have more cards in the deck, they would be the same as some other cards and they would be superfluous. Another example would be a game of chess where a player would like to have more than 16 pieces. Then, they would be the same as one or more of the pieces already present and they would be superfluous.