What are indexes in MongoDB and why are they important?

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An index is a separate B-tree data structure that lets MongoDB find documents without scanning the whole collection. Trades disk + write speed for read speed. _id index — created automatically and cannot be dropped. Single-field — one column; createIndex({ field: 1 }) ascending or -1 descending. Compound — multiple fields; serves queries on the leftmost prefix. Multikey — auto-built when indexing an array field. Special types — text, geospatial (2d/2dsphere), hashed, wildcard, TTL. Cost — slows writes and uses RAM/disk; index only what you query. Know index types: single, compound, multikey, text, TTL, geospatial, hashed. explain() is the key debugging tool (IXSCAN=good, COLLSCAN=bad). Compound index field order matters (supports queries using left-to-right prefix). TTL for auto-expiring sessions/caches. Unique for email/username constraints.