Nitrate is the far less toxic end-product of the nitrogen cycle, which builds up over time and is removed by regular partial water changes and (partly) by live plants.
Some nitrate is normal and even feeds plants, but letting it climb stresses fish and fuels algae. Controlling it is exactly why we do routine water changes rather than just topping the tank up.
A steadily rising nitrate reading between water changes is your tank telling you how heavily it is stocked and fed — it is a useful maintenance gauge, not an emergency like ammonia or nitrite.