This is Kirschoff's law - No charge, and so current, accumulates at a node (junction between wires). So the sum of all the currents going in must equal to the sum of all the currents going out.
This is Kirschoff's law - No charge, and so current, accumulates at a node (junction between wires). So the sum of all the currents going in must equal to the sum of all the currents going out.