Um, it is in the manual.
But, it means that the TPMS is indicating a low tire pressure.
BTW - was it flashing? - That would mean that the TPMS either has a malfunction or one/more of the TPMS has gone dead. There is a battery, but it is a pain to try to replace. Best to have a tire place to replace it. Then the TPMS system in the car will need to "relearn" all sensors.