A shortcut to a record
To find pages about gardening in a book, you can use its index instead of reading every page. A database can use an index in a similar way. If an online shop indexes order numbers, it can find a particular order without checking every order.
The index is built from selected details, such as order numbers or dates. It does not change the orders themselves.
Why not index everything?
Each index takes up space and must be updated when records change. Too many indexes can slow down adding or changing records. An index helps only with searches or sorts it was built for, so teams choose them based on how people use the data.