Indices contribute to faster query output retrievals. This sect1 will explain the different types of indices available, as well as when you should use one type over the other.
Indices are used to enhance database performance, but if overly used, then it could inhibit database performance. Upon performing a search query, the database needs to check every row in the table. If you have large tables, then this can take a long time for it to perform the check. An index allows the database to quickly thumb through the table rows and quickly find the rows that you're looking for.