To ensure the validity of data in a database, you need to formulate checks to define valid and invalid data, and design rules to which data must adhere (also known as business rules). Together, checks and rules become constraints.
Constraints that are built into the database itself are more reliable than constraints that are built into client applications or that are spelled out as instructions to database users. Constraints built into the database become part of the definition of the database itself, and the database enforces them consistently across all applications. Setting a constraint once in the database imposes it for all subsequent interactions with the database.
In contrast, constraints built into client applications are vulnerable every time the software changes, and may need to be imposed in several applications, or in several places in a single client application.