In C++, std is a standard namespace which contains the standard library functions such as input/output functions (cin, cout), containers(vectors,map) and algorithms i.e., find, sort. We use it in our programs as following: 
int n; 
std::cin &gt;&gt; n; 
std::cout &lt;&lt; n; 
But instead of using std each time we can add the using namespace to program before main to access its functions directly without using std each time. 
int n; 
cin &gt;&gt; n; 
cout &lt;&lt; n;

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In C++, the 'std' namespace encompasses standard library functions like input/output functions (cin, cout), containers (vectors, map), and algorithms (find, sort). Rather than prefixing these functions with 'std::' each time, programmers can use 'using namespace std' to simplify code and access functions directly.

Use of std

To add to this;
Some functions and classes of the standard namespace ar me inside a nested namespace (for instance <code>std::chrono</code> and <code>std::this_thread</code>).
Using the keyword <code>using namespace std</code> is a scoped statement. That means it works in the scope where you place it. If you place it in the global scope, you can use it in the complete C++ file. If you put it in a function or method, you can use it in that function or method only.
Some people, including me, recommend to not use <code>using namespace std</code> at all. The reason for this is that it can lead to confusion if you use functions that are available in multiple namespaces.