Could be a number of things, but I believe it's most likely an overflow alarm for a sump pump that moves sewage uphill from a tank on your property to a main sewage line.
If the tank is overflowing, a fix could be as easy as opening up the tank and removing paper or sanitary napkins stuck in the pump inlet. If needed, a replacement pump can be bought at larger hardware stores for a couple of hundred dollars max. Wear fluid resistant work gloves.
We once had a home with one of these sump systems because the developer was too cheap to run proper sewage lines. I swore never again...
ETA: or your septic tank is full, like the previous post said.
This post was edited on 1/3 at 5:08 pm