NDCG
Normalized Discounted Cumulative Gain is an evaluation metric that measures the quality of a ranking system by penalizing highly relevant items placed low.
Think of It Like This
Like scoring a librarian higher if they hand you the exact book you want first, rather than handing it to you tenth.
NDCG assumes that highly relevant documents are more useful when appearing earlier in a search engine result page. The 'discounted' aspect logarithmically reduces the score contribution of items found deeper in the list. The final score is normalized against the ideal perfectly sorted list, yielding a value between 0 and 1.