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:

I_in₁ + I_in₂ + I_in₃ + ... + I_{in_n-2} + I_{in_n-1} + I_{in_n} = I_out₁ + I_out₂ + I_out₃ + ... + I_{out_n-2} + I_{out_n-1} + I_{out_n}