In layman's terms, an in-memory database is simply a repository of data that does not rely on disk storage. Instead, the data is stored in the main memory, closer to where the actual processing takes place.
This is not a new technology. However, where an in-memory database may have been an optional consideration in the past, it’s now considered essential for enterprises that manage a lot of data and rely on complex analytics. But smaller companies and nonprofits now have access to a lot of data as well, and if they want to remain agile, they need to be able to analyze that data in real-time.