Nitrite is the second toxic stage of the nitrogen cycle — produced when bacteria break down ammonia — and like ammonia it should read zero in an established tank.
Nitrite stops fish from carrying oxygen in their blood, so a nitrite spike can suffocate fish even when there is plenty of oxygen in the water. The second group of beneficial bacteria converts it into nitrate.
A nitrite reading above zero usually means a tank is only part-way through cycling, or the bacteria have been disturbed. Water changes buy time while the colony catches up.