Check constraints are a powerful tool in SQL for maintaining data integrity. They allow you to define rules that must be met when inserting or updating data in a table. These rules are specified using a WHERE clause-like syntax within the constraint definition. This ensures that only valid data is stored in the database, preventing inconsistencies and errors. For example, you can enforce that a price column always holds positive values or that a date column contains dates within a specific range. This prevents accidental or malicious entry of incorrect data. By defining these constraints, you can ensure that the data in your tables accurately reflects the real-world entities they represent. This is essential for maintaining the reliability and trustworthiness of your database.