Durability guarantees that once a transaction has been committed, it will remain committed even in the case of a system failure (e.g., power outage or crash).
Durability is one of the four ACID properties (Atomicity, Consistency, Isolation, Durability) of transactional systems. It depends on successful commit semantics from Atomicity, and together with Consistency and isolation it forms the ACID transaction guarantees.
The everyday meaning of “durable”, a product that stays useful for a long time, is a different quality: see longevity.