“using namespace” lets the IDE know which namespace any commands issued is calling from, saving the coder time while coding. This is problematic in larger programs as different libraries could have similar namespaces, resulting in clashes.
If the namespace is defined as std from , a subsequent cout would be understood as std::cout. If however, another library also has namespace std, the program would not understand which library it is calling namespace from.
That’s what I understood from the lesson.