They are probably talking about the capacity bar display on the dashboard. 6 remaining capacity bars out of the original 12 means that the car has less than 50% of its original capacity left, with a range of no more than 35 miles in typical driving (much less in cold weather) and I suggest you pass on it. I will link my used Leaf buying guide here shortly. Please read it!
Scarlet Ember 2018 Leaf SL W/ Pro Pilot
2009 Vectrix VX-1 W/18 Leaf modules, & 3 EZIP E-bicycles.
BAFX OBDII Dongle
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.