Cache
A fast temporary copy of data kept close at hand, so an application answers repeat requests without redoing the work.
What it tells you about a developer
Caching experience is a performance and scale signal. Small apps answer every request straight from the database. A developer who has added a cache has worked on something with enough traffic that speed mattered, and has dealt with the hard part: keeping the fast copy honest when the underlying data changes.
A cache holds copies of recently used data in memory, where reads take a fraction of a millisecond, while the database keeps the permanent record. Caching makes apps feel fast and shields databases from repeat traffic. Redis is the most common caching tool. The classic hard problem is invalidation: knowing when a cached copy has gone stale.