What are Precision@K and Recall@K, and why are they always read together?

Precision@K is the share of relevant items among the K shown: how many of ten cards landed. Recall@K is the share of all relevant items you managed to retrieve. Precision governs the quality of the shelf, Recall the completeness of retrieval. Read separately, both are trivially gamed: showing one guaranteed favourite gives Precision@1 = 1 with negligible Recall, dumping the catalogue gives Recall = 1 with zero Precision. In a cascade they split by stage: candidate generation is tuned for Recall, ranking for Precision and NDCG.