sewage pump high water alarm
I assume this pump is in a pump chamber that leads to your septic system. If not, please tell us where the pump is.
The alarm comes on when the level in the pump chamber exceeds the preset alarm level, which is typically about 6 inches below the top of the pump chamber. The purpose of the alarm is to warn you that the pump is not working, so you will not continue to flush sewage into a full tank, eventually overflowing the tank onto the ground. My neighbor had the same problem, it was a broken pump, but he unfortunately ignored the alarm until there was sewage all over his front lawn.
As noted, you need to open the manhole to the pump chamber and determine if the pump is functional. If not, get it replaced of course, although it could be the float as previously noted, or even the wiring from the house to the pump. If the pump chamber is cracked, groundwater could be flowing into the pump chamber, although the pump should be able to pump out a limited amount of groundwater, as previously noted.