This says that, in a closed electrical circuit, current cannot accumulate at a junction. For an equation, take a look at this diagram:
          a ->       <- b      
                    |
                    |   ^
                    |   c
                    |
                    |
                    |

where a, b, and c represent the magnitude of current (Amps) flowing into the junction along the three wires. Kirchoff's Current Law dictates that a + b + c = 0.