This is a none-Pb (lead-free) low-power surface mount Zener diode, which is typically used to protect circuits from high voltages. It has a zener voltage of 5.1V, with a current of 1.08mA and a voltage tolerance of +/- 5%. Its power dissipation rating is 500mW, with a junction temperature of 150°C and its maximum reverse voltage rating is 6V.