DHT stands for Distributed Hash Table. In BitTorrent, Mainline DHT lets peers find other peers for a torrent without needing a traditional central tracker.
DHT makes BitTorrent more decentralized. Instead of asking one tracker server where peers are, clients ask the network itself. That design helps with resilience and supports trackerless torrents.
No. DHT is a network mechanism. A torrent search engine is a separate website layer that may index information gathered from the BitTorrent ecosystem.