first your fish produce ammonia as they breathe and as they crap,
this amonia is broken down into nitrItes by bacteria in the filter, this is less harmfull to fish but still not gopod to have in your tank
nitrItes are then turned into nitrAtes by more bacteria this is much less harmfull to fish and can be4 removed with a water change,
how long has your tank been set up as you should only see nitrites is someing is wrong or the tank is still cycleing