The SQL command SELECT is used to retrieve data from a database. It allows users to specify which columns of data they want to see and from which table. For example, a query like `SELECT name, age FROM employees` will return the names and ages of all employees in the employees table. In addition to selecting specific columns, SELECT can also include conditions to filter results. Using the `WHERE` clause, users can limit the data returned based on certain criteria. For instance, `SELECT * FROM employees WHERE age > 30` retrieves all information for employees older than 30 years.