Think of Ruby as the raw materials needed to build a house. Rails is a template or blueprint that *automatically builds* many of the common parts of the house that all houses need, without you having to do all the planning or building.
You’re gonna need windows, so Rails makes spaces in your walls for windows.
You’re gonna need a furnace, hot water heater, electrical wiring, and a roof over the house, so Rails takes care of all of that. Every house needs those things. There’s no need to waste time building it all on your own.
You get to design and build all the fun stuff on top of the framework Rails has created for you.
The layout of the rooms, the shape of the house, the kinds of windows, the bathroom fixtures, type of flooring, lights and lamps, kitchen cabinets and so on. That's your part.
In summary: Rails builds a scaffold (framework), upon which you can flesh out your house...er...project.
All the boring, monotonous steps have been taken care of for you.
