what is a mock-up?

A mock-up is a model made for demonstration, design-evalution, etc. When talking about book covers, a mock-up is a display of you cover on a book.

This is how mock-ups look that can be used in Photoshop. You basically drop your image in there.

That is not possible on any other free design app that I have seen.

So unless you want to dreadfully change the perspective of your cover and try to fit it exactly into the mock-up you need an alternative.

diybookcovers.com

The only good free mockup option I have encountered is diybookcovers.com

You can choose from like 30 different layout options, this one is my favourite:

There are even some where you can include your book jacket:

All you need to do is chose your design, upload your cover and done.