Atlas Kit is a React-based UI library / toolkit that we use in our cloud products and that we make available to third-party app developers to incorporate into their Atlassian apps. The UI library is built according to the Atlassian Design Guidelines (ADG). Atlas Kit has a whole bunch of components and APIs for building rich designs and interfaces that are consistent with our products’ design and experience.
For example, if you want to render a drop-down menu, you’d use the Dropdown menu component. In the docs, you can see examples of a rendered drop-down, as well as example code for generating it. This is true across all (or at least most) of the other components, too.