OpenCv (Open-source Computer Vision) is a library that you can use to modify, manage, up to take a decision from images on computer. It was designed for computational efficiency and with a strong focus on real-time applications.

For short, the following is (we may call it) a “hello world” application that we use OpenCv’s library to display an image on a computer screen. Here, I use OpenCv 4.1.1 version with C++11 compiler running on lubuntu 16.04 operating system. 
#include <iostream>
#include <opencv4/opencv2/highgui/highgui.hpp>
using namespace std;
using namespace cv;
 
int main()
{
 Mat im = imread("../fruits.jpg");
 namedWindow("Hello World");
 imshow("Hello World", im);
  
 cout << "Press 'q' to quit..." << endl;
  
 while(static_cast<char>(waitKey(1)) != 'q');
 return 0;
}
For simplicity, I’ll break the code down into chunks to explain.
Mat im = imread("../fruits.jpg");
This creates a variable im of type Mat (we don’t need to write cv::Mat here because we have used using directive: namespace cv; making names from cv directly accessible).
The argument of the function imread(), ../fruits.jpg where ‘../’ indicates the location of the file, in this case in ‘build’ folder and fruits.jpg is the file name. 
namedWindow("Hello World");
imshow("Hello", im);
Figure 1. Result of the code
The first line creates a window called Hello World (note: Hello World is also displayed in the title bar of the window) and then shows the image stored in im in the window. And the second line waits for a key event infinitely; when user press ‘q’ the program will exit. This function returns an ASCII code of the pressed key or -1 if no key was pressed before the specified time elapses. Note that waitKey() works only if an OpenCV GUI window is open and in focus.

That’s it!, our “hello world” program for opencv. If there is something incorrect please comment for me to correct or if there is something you don’t understand please ask and share :)