A Foundation Framework is a that is built prior to any application that are built on top of it. The idea is that you analyze the needs of the various applications that need the framework, then you build the framework. Once the framework is complete you then build applications on top of it. The point is that the framework really needs to have a stable API before you start work on the applications, otherwise changes to the framework will be hard to manage due to their knock-on effects with the applications.
While this sounds reasonable in theory, I've always seen this work badly in practice. The problem is that it's very hard to understand the real needs of the framework. As a result the framework ends up with far more capabilities that are really needed. Often its capabilities don't really match what that the applications really need.
Contrast this with HarvestedFramework