Nitrate is the last step in a tank's denitrification process. Unlike nitrites or ammonia, nitrate is far less toxic to aquatic life. However, a high level of nitrate will indicate build-ups of fish waste, decaying food, or a problem in your filtration system. High nitrates are often a precursor to poor water quality and can result in significant unwanted algae growth.
Keeping Nitrates low is one of the primary goals of fish keeping regardless of if you keep saltwater, freshwater, or ponds.
API's Nitrate test kit is what we use on a daily basis on our own tanks and ponds. This test kit works on both fresh and saltwater.