The concept of Azure Cache for Redis is based on a software Redis.
Generally, it is used as a cache to improve the performance and scalability of systems that rely heavily on backend data-stores. If your application has a really large datastore in the backend using Redis cache will definitely help in improving the performance.
This is done by temporarily copying frequently accessed data to fast storage located close to the application. With Azure Cache for Redis, this fast storage is located in-memory with Azure Cache for Redis instead of being loaded from disk by a database.