The endl keyword moves the cursor to the next line after printing. Without endl, all output appears on one line, which can be hard to read. Add cout << "Second line" << endl; after the first cout statement.
Run the program. You will see two separate lines because endl moved the cursor down before printing. You can also use "\n" inside a string instead of endl to create new lines. Both work the same way.
For now, use endl in all your programs.