Requirements are product features that must be present. A Design is one of many potential ways to satisfy that set of Requirements.

For example, a requirement might be "the user shall not be exposed to hazardous voltages (defined elsewhere) when servicing the equipment."

A possible Design solution might be "provide cover interlock switches so when the covers are opened, all voltage supplies are disconnected." or "software monitors a cover switch, and when that particular cover is opened, a command is sent to the power controller to disconnect power to anything that is reachable from that opening."

Which of the two (or other) design options is chosen, is a Design Decision, but they are means to an end, that end being Satisfying The Requirement.