Electric current is produced by the flow of electrons in an electrically conductive material. For current to flow, the conductor — for example, a copper wire — must be connected to a voltage source. Due to the potential difference, the electrons are pushed from the negative pole of the voltage source to the positive pole.
Since an electric circuit must always be closed, meaning no charges can escape, the current inside the voltage source flows from the positive pole back to the negative pole.
In this book, for simplicity, the so-called conventional current direction is used. In this convention, current flows from the positive pole to the negative pole, i.e., opposite to the physical flow of electrons.