The wiki for Continuous Integration pretty much explains it.
Build tools relate to what is necessary to build your entire project. If your project is 100% pure python then you probably don’t have any build tools as its all interpreted scripts. You would keep committing to the project, and it would keep building (if building is neccessary), and testing.
Compiled languages (or if your python project needs to be packaged somehow to be tested) would require build tools to produce a program that can be run and be tested by the CI system.
If, for instance, your project were C++, you might be using
make as your build tool. When you commit new code,
make has to be run to build the project and be tested.