There isn't much to explain, it is a game window to play pool. Whenever a ball goes in a hole it disappears. You can only hit the white ball with the stick and the velocity of the ball depends on how much you charge the stick, the longer the faster.
Simply enter the directory in the terminal and do make. If you haven't cloned the repository the instructions will be
git clone https://github.com/Jerry-Master/Billar.git
cd Billar
mkdir
make
It is possible that you will need to execute it as administrator, in that case just use sudo make. Don't worry about that, it is only for the creation of the obj directory.
In the command line do ./main.exe. A window will appear with the pool table.