A marketable product is something that people will need or want so badly that they will pay for it.
To create that product, you have to make sure there is a true desire for it in the market. Save yourself a lot of trouble by doing your research from the get-go to establish who would need your product and why. What you sell doesn’t have to be a completely new idea, it just has to do it better, more easily or for less money than anything else that is out there.