C++ is the other of the two most widely used programming languages in particle physics. It is typically used for computationally intensive work and many HEP and ATLAS tools require a working knowledge of it.
A thorough introduction to modern C++ is available here:
The tutorial includes are some advanced C++ features that are not necessary for getting started. For now, we recommend that you focus on lessons 1-9, 12 and 14.
If you want to do a deeper dive into C++, you can find a very thorough reference guide here:
https://en.cppreference.com/w/
This is not necessary for this tutorial, but is very useful to keep bookmarked for future reference.