Reranking is a second pass after initial retrieval. The system takes a set of candidate results and scores them again for relevance to the user question.
In practice
Reranking can improve RAG quality because the first search step may return many near matches. A reranker helps choose the chunks that best answer the exact query.
What to watch
Reranking adds cost and latency. It also cannot recover information that the first retrieval step never found.