Nitrite is a substance that is toxic to fish and invertebrates.
It is a misconception that once a tank is through its cycle (first 3 or 4 weeks) that nitrite has vanished.
Many nitrite test kits suffer from interference by amines (such as naturally occurring and essential amino acids) resulting in a too low measured value especially if the nitrite concentration is relatively low.
Nitrite is converted to nitrate in properly cycled aquariums. The formed nitrate is transformed into harmless nitrogen gas (=denitrification). This happens in areas deprived of oxygen within live rock and miscellaneous filter materials.
Before it can be transformed to nitrogen gas the nitrate is first converted back to nitrite. When for whatever reason this particular cycle can not be completed, toxic nitrite can be present in significant concentration and readily be detected by test kits that do not suffer from amine interference.