Index is a feature that speeds up searching and sorting in a table based on key values and can enforce uniqueness on the rows in a table. The primary key of a table is automatically indexed. Some fields can't be indexed because of their data type. There are two type of indexing - single field and multifield.
An index speeds up queries on the indexed fields as well as sorting and grouping operations. For example, if you search for specific employee names in a LastName field, you can create an index for this field to speed up the search for a specific name.
If you think you'll often search or sort by two or more fields at a time, you can create an index for that combination of fields – multifield.
However, indexes can slow down some action queries such as append queries.