SQL Server uses indexes to quickly locate data without having to search every row in a table every time a query is run. Indexes can be created using one or more columns of a table, providing the basis for both rapid random lookups and efficient access of ordered records.
For example, if you had a table of customer orders, you could create an index on the customer name and order date columns. This would allow you to quickly find all orders for a particular customer, or all orders placed on a particular date.