Cardinality is a crucial concept in SQL optimization. It represents the number of rows in a table or the result of a query. A high cardinality table (many rows) will likely require more resources to process than a low cardinality table (few rows). Knowing the cardinality of tables and query results allows the database management system (DBMS) to choose the most efficient execution plan. For instance, if a query involves joining two tables, knowing the cardinality of each table and the join condition can help the DBMS estimate the size of the result set. This estimation is vital because it allows the DBMS to select the most appropriate join algorithm (e.g., nested loop join, hash join) and avoid unnecessary processing. A query involving a table with a high cardinality might benefit from indexing or other optimization techniques to reduce the number of rows processed. In essence, cardinality provides a measure of the data volume involved in a query, enabling the DBMS to make informed decisions about how to execute the query efficiently.