Responsiveness means that the user interface adjusts to changes in screen or window size. While Plasma and especially Kirigami are highly responsive, this is not a replacement for optimized convegence. It is not supposed to be used as a replacement for different UI and UX for different interaction methods (mouse, touch, pointer, remote, …) or different form factors like mobiles and televisions. Instead, it means to adapt to different screen sizes in the same form factor class, like different window sizes and screen resolutions in desktops and laptops.
Examples of responsive behavior:
Components can display more or less content, depending on the available space.
Margins between elements can shrink and grow
Multi-column navigation layouts, grids and grid-like layouts can change the number of displayed columns
Images can resize
Although elements can resize vertically, horizontal or in both directions, it is recommended that row-like elements such as menu bars only shrink or grow horizontally.
Action bar shrinking only in the horizontal direction.
Keep in mind that not all controls or parts of a layout can and should be responsive. Often it is more important to retain the size for usability, familiarity and alignment and aim for optimized convergence by selecting different navigation, command and content patterns for different formfactors.
It is recommended to test your user interface against the most common screen sizes of the targeted form factor.