If you have the proper amount of air in the tires it is not serious. It is annoying in that you have to look at a meaningless warning. There are several possible causes of the problem, i.e. the battery in the sensor could be dead. Of course there is no way to replace the battery in the sealed unit. You have to buy a new sensor, instead.
A trip to a dealer or a friend with a GS-911 may tell you more. If you have to replace a sensor you'll have to go to a dealer to pair the new sensor with your bike.