The list on the left displays all slots your ship chassis contains for placing components. All ships share some key component slots—reactor, engine, front/back armor, capacitor, booster, etc. -- and also contain specific slots, like weapons, ordnance, and cargo holds. Double-clicking a slot will expand it to show all components in your inventory that can fit in that slot; selecting a component from this list will show it in the Selected view to the right, allowing you to quickly compare its stats to the Installed component. Finally, double-clicking a component (or pressing the Load button) will unload an installed component and replace it with the selected one, provided your ship has enough mass to hold the new component.