Cobalt is an open-source application platform for Kobo e-readers, allowing users to install and run apps on their devices. The platform includes a launcher, app store, and runtime, and supports the development of new apps using a Rust SDK. Users can install Cobalt on their Kobo Clara BW devices and access a range of apps, including an arXiv reader, Sudoku, and a Hacker News client. The platform is designed to be safe and recovery-friendly, with verified signatures and independent app releases.