In fine restaurants, the chef is usually well paid for his craft. However, it still ends up being a thankless job. He/she keeps cranking out masterpiece after masterpiece, and yet no one appreciates his labours. If one enjoys the meal, truly enjoyed it, you can tell the waiter to send your compliments to the chef, to thank him for his care in preparing your food. It's a nice thing to do.

The phrase has been extended through slang to mean appreciation for anyone doing some sort of service. Used when saying it in a place/time that that person would not normally hear about it. It pops up every once in a while on the Internet.