H2 Database Engine is an open-source, Java-based relational database management system. It is designed to be fast, lightweight, and highly versatile, making it suitable for a range of applications including embedded systems, desktop applications, and more. H2 offers features such as in-memory storage, the ability to run as an embedded or server-based database, full transaction support, and compatibility with SQL standards. With its minimal footprint and ease of integration, H2 is a popular choice for developers looking for a robust database solution that can be easily embedded within Java applications.