Nitrite is the first byproduct of a tank's denitrification cycle. Like ammonia, nitrites are toxic to aquatic life. Spending a few minutes each week to measure the concentration of Nitrite will help you understand the health of your tank. It will allow you to know where your water is in the denitrification cycle and alert you to problems such as overfeeding or insufficient filtration.
API's Nitrite test kit is what we use in our own tanks. It is suitable for either fresh or saltwater.