https://www.engineersedge.com/battery/battery_over_charging.htm
In essence it means that the battery has been discharged too much/too low and may not now be capable of holding a full charge so well.
So that red light has done you a favour as it's telling you that it has to keep charging up the battery more regularly than it otherwise should do, which indicates that the battery doesn't hold a charge as well as it should. Time to replace the battery I reckon (but sadly not the boat and tackle as @Saintly Fish would have you believe!).
Edited to add: there's a chance that the wiring between the charge controller and the battery has some corrosion somewhere, causing unexpected resistance, so before you go to the expense of replacing the battery, check the wiring & joints/connectors and clean them up if needed.