Index
In plain terms
A database structure that makes looking up rows much faster.
Going deeper
An index keeps a sorted lookup of column values pointing to rows, so queries find matches without scanning the whole table. It speeds reads but adds storage and slows writes a little.
Under the hood
B-tree indexes suit range queries; hash indexes suit equality; composite indexes depend on column order. Over-indexing slows writes and bloats storage.
Examples
CREATE INDEX on email, primary-key indexes, covering indexes, full-text indexes.
Think of it like...
Like the index at the back of a book: instead of reading every page to find 'mitochondria', you jump straight to the listed pages.