What does "batteries included" mean?

"Batteries included" is a reference to battery-powered toys that either come with or without batteries. Often at the end of old toy commercials in the 90s you'd hear the disclaimer "batteries not included", so you had to remember to buy batteries along with the toy.

In software development, especially when talking about Web Development Frameworks, "batteries included" refers to how much the Framework provides for you. Frameworks that provide a lot of functionality are said to have the "batteries included".

Rails is probably one of the most "batteries included" frameworks out there, which is by design. As compared to ExpressJS which is a self-proclaimed minimalist framework.

Usually, when people talk about the pros and cons of a "batteries included" framework like Rails, they also talk about whether or not the Framework is "opinionated".