A sump pump check valve is located in your sump system’s discharge line. The purpose of it is to prevent the water that was just pumped out from flowing back into the the sump pit when sump pump turns off.
Sump pump check valve is an important component of the basement sump pump system. Without it, the system will undergo extensive damage. It can be noisy but it is needed ( you may hear noise when the sump pump check valve slums shut under the pressure of water flowing back.)
The most common problem with it is a leaking valve. Sometimes, people replace the whole failed sump pump, when in reality they could keep it and replace only the sump pump check valve.
If you decided to replace a check valve, I suggest that you don't buy cheap corrugated drain kits and use 1.5" PVC instead (which is actually even cheaper than the cheap corrugated kits). The check valves have a lip that the PVC fits into perfectly, while corrugated kits are harder to fit.
Above is a picture of typical sump pump check valve that can be bought at the Home Depot. Reviews seem to provide a range of opinions: ratings vary from one to five stars. A professional plumber like me would likely use products from professional lines. We don't like 3 a.m. calls from the clients an we take sump pump installation very seriously. We provide warranty for our work.
The placement of the check valve is important. If your discharge line runs to your basement ceiling ( which is common) and you place it too close to a submersible sump pump, then when when the pump is not working, all the weight of the water in the discharge line is pushing the the check valve down. So it is keeping it closed. However, as your sump pump ages, it will have a hard time pushing against so much weight when it starts pumping. Best height ( unless owner's manual states otherwise) is two to three feet off of the top of the sump pit. This way your sump pump motor doesn't have to work as hard as when your check valve is in lower position. It will open easier due to the reduced weight of water above it.
If you are not sure how to replace or diagnose sump pump check valve, please call a plumber. This is not an expensive repair; however, if you get caught with raising water in your basement while trying to resolve it by yourself, you might wind up doing basement cleanup from water damage.