Briefly:
"Ruby on Rails is an open-source web framework that's optimized for programmer happiness and sustainable productivity. It lets you write beautiful code by favouring convention over configuration."
The less technical description might be:
"Ruby on Rails gets your website up and running quickly by following simple instructions to build your application for you. Unusually, it combines out-of-the-box productivity with the flexibility you need to solve the detail."
- Bill Horsman
What that means to you is: