A program has line numbers so we can track the current instruction a computer is processing.

When you pretend to be a computer and execute a function call, you can go to the line number where that function is defined and start processing the instructions there.

When you reach the end of the code in the function, where the indentation has changed, you navigate back to the line number where the function was called.

Each time a function is called, the computer navigates to the line where the function is defined and the same code is executed.

The flow of the program is how the computer navigates the code, which line of code does it move to next.

Some instructions, such as calling functions, change the location of the next instruction to be executed and control the flow of the computer code.