This is my interpretation of the lecture, feel free to comment!
If using namespace std; is used in the code, there is a chance of a namespace clash. This means that more than one item in the std namespace have the same name. This risk increases as several libraries are included in the code.
To try and lower this risk as much as possible, it might be a good idea to just say: ‘using std::cout’ and 'using std::endl"