The line means they are exporting (or importing) at the moment.
But it doesn't mean the supply (demand) will be high enough to ensure nice profits. That is something you have to find out personally at the station.
(Another note - not every station/base in the system has the same economy as the system itself. So it may take a while to find the right station. You can see the main import/export commodities in the system map, if you have the data)