It means the power supply detected is lower than what is needed to charge the tablet properly with the tablet on so it is charging at a lower speed. This can happen if you are using your computer to charge, if you use a charger different that the official charger that came with the tablet, if you have a problem with your power source, if the charger or cable is defective, if there is a problem with your charging port, or even a firmware problem.
So, things to try depend on your current situation. If charging from a computer - use a wall outlet. If not using the Samsung charger that came with the tablet, use that charger. If you are using that charger, try a different wall outlet, then try a different cable, then try a different charger with the same or greater specs as the Samsung charger.
Of course, if you have not turned off your tablet and turned it back on, do that as well. Does that tablet have a removable battery? If so, remove the battery for a minute. Also make sure the charging port is clean and no bent pins are seen. Lastly, try a factory reset. If all the above doesn't make a difference, then since it is brand new, exchange it.