Kasoori methi are dried fenugreek leaves (Trigonella Foenum Graecum) . They are used in Indian cooking and taste similar to a combination of celery and fennel with a slightly bitter bite. Typically the leaves are crumbled and sprinkled over meat and vegetable curries before serving.
Look for fresh methi leaves in Indian grocery stores or use dried methi. Dried methi leaves are very pungent. Look for them in small boxes or bags.