Redis

Redis is an in-memory cache that is used to cache data structures in server memory. Sometimes Redis is referred to as an ‘Object Cache.’ Redis can be used by PHP and by extension, WordPress, to cache database lookups. WordPress sites can have huge database queries that can quickly overwhelm memory and data links between application and database servers, as well as high TTFB times.

Redis is often implemented to cache database lookups (queries) so they are available in server hardware memory, which is much faster than accessing the same data from the hard disk array.

Further reading:

Wikipedia: https://en.wikipedia.org/wiki/Redis