A-level Physics (Advancing Physics)/Current/sigma

That's the Greek letter Sigma. It means 'sum of', so Kirchoff's First Law is stating that the sum of all the currents going into a point equals the sum of all the currents leaving the point. So:

Iin 1 + Iin 2 + Iin 3 + ... + Iin n-2 + Iin n-1 + Iin n = Iout 1 + Iout 2 + Iout 3 + ... + Iout n-2 + Iout n-1 + Iout n