Good search returns relevant results first:
TF-IDF: Term Frequency times Inverse Document Frequency. Rare terms in a document score higher.
BM: Improved TF-IDF. Standard in Elasticsearch. Handles document length better.
Boosting: Increase weight for certain fields. Title matches score higher than body matches.
Personalization: Factor in user behavior. Items you've viewed, purchased, or clicked rank higher.
A/B testing: Measure click-through rates, search abandonment. Small ranking changes have big impact on user experience.