CashScript is a high-level programming language for smart contracts on Radiant. It offers a strong abstraction layer over Radiant' native virtual machine, RadiantScript. Its syntax is based on Ethereum's smart contract language Solidity, but its functionality is very different since smart contracts on Radiant differ greatly from smart contracts on Ethereum. For a detailed comparison of them, refer to the blog post.
If you're interested to see what kind of things can be built with CashScript, you can look at the Showcase or Examples. If you just want to dive into CashScript, refer to the Getting Started page and other pages in the documentation.