Mockups used to represent the work to the owner in a realistic environment. It is a kind of presentation that allows us to visualize the result in use or real life. Most of the mockup work is done in PSD Photoshop document format.
For instance, we order someone to design a logo. After creating the logo designer needs a mockup to present the work to the company owner, visualize how it will look in real life.
There is a massive variety of mockups uploaded on different websites. Some of them for offering free mockups, and some of them are offering paid.
You can find mockups about logos book covers, posters, advertisements, billboards, table, laptop, mobile, stationery, business cards, letterheads, cars, vans, and many other things that could be needed for a designer to present their work.