A compute capsule's runtime environment is built on top of a base environment curated and prepared by Code Ocean to ensure consistency and reproducibility. A base environment consists of an operating system (most commonly, Ubuntu Linux 16.04) and a minimal set of accompanying packages. Among these packages is usually a language interpreter, compiler, and/or framework.
Every new capsule will begin with a default, language-specific base environment. You can pick among different base environments via the dropdown list in the Run Environment screen.
(Switching between different base environments is also how one accesses a GPU.)