When you make a call to using namespace <some_namespace>; all symbols in that namespace will become visible without adding the namespace prefix. A symbol may be for instance a function, class or a variable.
E.g. if you add using namespace std; you can write just cout instead of std::cout when calling the operator cout defined in the namespace std.
This is somewhat dangerous because namespaces are meant to be used to avoid name collisions and by writing using namespace you spare some code, but loose this advantage. A better alternative is to use just specific symbols thus making them visible without the namespace prefix. Eg:
#include <iostream>
using std::cout;
int main() {
cout << "Hello world!";
return 0;
}