The Components panel shows all the Components that are part of the current Blueprint:

Components are ready-to-use objects that can be added to Blueprints. To do this, click on the green Add Component button of the Components panel. A Blueprint can be created with various features just by using Components.

The properties of a selected Component can be edited on the Details panel and the visual representation of some Components can be seen on the Viewport panel.

Static Mesh, lights, sounds, box collision, particle system, and camera are examples of Components found in the Components panel.