Freight Tonne Kilometres (FTK) measures actual freight traffic. Some airlines disclose Cargo Tonne Kilometres (CTK) which explicitly includes unaccompanied baggage and mail, avoiding ambiguity.
It is the equivalent of RPK for freight.
One FTK is one metric tonne of revenue load carried one kilometre. The sum of FTKs for every flight stage flown by every aircraft over a period is the FTK of an airline over the period.
Freight Tonne Kilometres is a measure of how much freight business an airline gets. Weight carried is not adequate measure because distance matters: carrying a tonne 10,000km is clearly a greater supply of transport services than carrying the same tonne 100km.
Cargo revenue per CTK (or FTK) is a measure of price (so the higher the better from the point of view of an airline)
The International Air Transport Association (IATA) produces monthly global traffic statistics which provide a benchmark for an airline's FTK growth. As most airlines are a mix od regional and global business it make take some effort to devise a fair comparator from these numbers.