Statistics for query optimization are objects that contain statistical information about the distribution of values in one or more columns of a table or indexed view.
The query optimizer uses statistics to create query plans that improve query performance. For most queries, the query optimizer already generates the necessary statistics for a high quality query plan; in a few cases, you need to create additional statistics or modify the query design for best results. This topic discusses statistics concepts and provides guidelines for using query optimization statistics effectively.
More information can be found on Books online: Statistics