It means the gears I have selected/the chain on. So for instance, on a slightly hillish terain if I have the chain on the 42t chainring and the 18t cog of the freewheel, that is the maximum amount of power I can handle for sustained periods of my rides which consist of 13 miles If I am on the 42t chainring and the 21t cog of the freewheel I can only sustain it for short intervals because my legs are going to be found by some pedestrian or bicyclist unlucky enough to find them on the path after I have keeled over and died .

If I am understanding you correctly... I should change out my freewheel before I change out the chainrings?

The rear is a 126mm hub which is standard I believe. IRD make a nice set with a tooth range of 13-32 for not a lot of money. Or a search for a suntour 6 speed would not be out of the question as my front deraileur is a suntour...

Something along these lines?

http://www.ebay.com/itm/SUNTOUR-free...4#ht_37wt_1359