It means satellite left and satellite right.
A flashing red LED should indicate that the satellites are being loaded. When they are charged, the indicator disappears. Make sure that the detachable speakers are correctly connected to the bar. After about 12 hours, both should be charged and the indicator should disappear.

Thanks for your answer, that makes sense but why does it say 0? The speakers have been docked to the station for days...

This only shows the battery status of the surround speakers.
This is especially relevant if you have set up the speakers separately from the soundbar and have not plugged them in to the
USB adapter to the power supply.