Fixed Price are projects in which you agree with the client on a fixed amount of payment, in regular time intervals. It doesn't mean that price will be locked to always be the same, rather it's a concept in which income is not dependable on the billed hours of your employees. Here's the simplest example.
This is a hypothetical example!
John's company has acquired a project deal with Company A. Company A proposed a 6-month deadline and a 60,000 € monthly payment. When John creates a project in Primetric, he needs to choose Fixed Price, and in the Budget -> Scheduled tab, he needs to fill in the amount of 60,000 € for every month in the Income section.