New comers to the C++ have generally no information about best and bed practices. We will look into why using namespace std is bad practice programming.
Namespaces a used to create code blocks where they do not collude each other and make things tidy and hierarchical. At the beginning by using namespace std seems not harmful bu in the coming stages of development bad things can be happen. As you know std is used by C++ default libraries if we use std too and create same function,class, variable with C++ default libraries we will get error about it.