Kasuri methi is dried fenugreek leaf used especially in South Asian cooking. A small amount, often crumbled before adding, gives sauces, breads and vegetable dishes a distinctive aroma. It is frequently added toward the end of cooking to retain that character.
It is not fenugreek seed: seeds and leaves differ in flavor, texture and usage. Fresh methi also brings moisture and bulk, so it cannot always replace the dried leaves in equal amounts.
Reference: Further reading.