It means libraries that were supplied by a "third party".... in another words someone other than the original Python publishers ("third" because it means other than you or Python publishers).
An example would be... NumPy (or the rest of SciPy for that matter)...
SciPy.org — SciPy.org[
^]
Edit: To add a bit of context, typically when you see instructions relating to third party libraries, it relates to libraries that have to be manually installed from non-standard locations.