Documentation Jargon

What does
"respond to width or height size changes" mean?

Width or height changes of what. An element? A window?

Respond how? When you drag a window larger or smaller?

Please clarify.

Well, context does matter, unfortunately you have not provided any context of where you are referring to in the documentation, so at a guess you are referring to flexible-layout

By default, Tumult Hype animations have a fixed width and height, and do not respond to window or viewport size changes. To make your animation responsive to size changes, you must first select the Width and Height Scale checkboxes in Hype’s Element inspector. Enabling these options allows your document to respond to width or height size changes as desired. The Width and Height Scale options are complimented by percentage fields which define how much of the containing window or div the Hype document should fill. The default value is 100%, which means the Hype document will expand or contract to fill the width or height of its container. Reducing this number restricts how the document will expand. For example, setting the value to 50% means the Hype document will only expand to fill half of the width or height of its container.

The first line in that gives you the context and the answer!.