Csound is a programming language for synthesizing sounds and creating compositions with those sounds.
It was one of the first computer synthesis applications and has had a long evolution, with the first version of Csound being released in 1986, and that being a descendant of MUSIC written by Max Mathews at Bell Labs in 1957.
If you love creating unique sounds, but wish sound design was more like FORTRAN programming, Csound might be the perfect environment for you. Although I say this jokingly, versions of MUSIC that Csound is descended from were written in FORTRAN, back when they still capitalized it.
It is widely used in academia as a teaching tool and due to the presence of methods for creating sound using pretty much every synthesis method ever created, it's great for exploring possibilities and learning about different techniques without purchasing expensive hardware or software, and has had significant use by musicians for creating evolving ambient soundscapes and experimental music.
Though there is a lot of documentation available on the web, much of it is not geared toward beginners. A number of books have been published that will help you get started:
The Csound website is at csound.com.