The High TDS alert will occur when the TDS of the water is above 2000ppm.
The system will adequately handle anything above 2500-3000ppm but the alert is intended to advise that you're not swimming in the absolute freshest water possible. High TDS also means you may not be getting the best cost effectiveness through power consumption and chemical levels.
If you are seeing this alert we would recommend having the water tested to see what current salt levels are and possible dilute the pool with freshwater where possible. Over time, the water will naturally dilute also.
More information can be found HERE.