Why we shouldn't use using namespace std in c++ code example

Example 1: using namespace std in c++

#include <iostream>

using namespace std;

int main()
{
    cout << "Hello World";
    system("pause");
    return 0;
    
}

Example 2: what is namespace in c++

//namespace is a declarative region to provide scope for identifiers
#include <bits/stdc++.h>

using namespace std; //including namespace std for cin and cout
//my custom namespace for variables and functions
namespace abc
{
  void fun()
  {
    cout<<"Hello world"<<endl;
  }
  int x=10;
}
using namespace abc;
int main()
{
  cout<<10;
  fun();
  return 0;
}