Does this have something to do with Ruby on Rails?
Is it something else?
Not any platform or language specific concept. It is a philosophy behind Information Architecture design.
Where the traditional approach has been to either simplify the model or use some sort of normalization to improve performance.
DDD takes the opposite approach, and tries to keep the data and entities in form of how its expressed in business lingo.
There's tons of debate on whether this is good or bad. The main arguments for it is it allows better decomposition of complex systems. Some of the main arguments against it is it can have a larger overhead, especially when using programming patterns like CQRS (which uses different data models for reads and updates) that have been known to cause large code and data storage overheads.