A database index is a data structure that improves the speed of data retrieval operations on a database table. It works like an index in a book, allowing the database management system to find and access data quickly without scanning the entire table. Indexes can be created on one or more columns of a table, enhancing the performance of queries that filter or sort data. Indexes can also consume additional storage space and may slow down data modification operations, such as inserts, updates, and deletes. Therefore, it's essential to balance the use of indexes to optimize performance while managing resource usage effectively.