A design mockup is usually an image or non-functioning preview of the design for a website.
Designers use mockups to propose to you what your final website would look like before a developers start to code it. Mockups are useful in ironing out the desired look and feel of the website fairly quickly with the designer while saving time and money.
This is because changing a design mockup is relatively fast and easy. On the other hand, changing a fully built website is far more complex, time-consuming and risks introducing bugs or errors.