The Client is the User Interface your Ground Station operators interact with. It's based on web technologies (with React and TypeScript at the core of its tech stack) and provides a framework for building re-usable widgets that get displayed in dashboards.
Dashboards are pre-defined layouts of one or more widgets within a single page that you can define for specific users. They are the pages that the ground station operators interact with and are selectable using the selector at the top of the user interface.
Widgets are reusable components of the UI that can you can place into dashboards to visualize Telemetry, send Telecommands, and so on.
By right-clicking into a widget and selecting "Configure", users can also reconfigure widget that supports that to, for example, choose the fields that get visualized within a graph widget.