What does the term "tanker" mean when used in regards to a passenger airliner?
Since the 1960's and 1970's when political hijackings were a problem, airplanes started to only carry the fuel required for the flight.  This means a typical fuel load will be something on the order of:


taxi fuel at the departure airport
fuel to fly and land at the destination at a given altitude and forecast winds
if an alternate is needed:


fuel to fly to the alternate airport

45 minutes of reserve fuel
perhaps a bit extra for contingency and to keep captains happy (e.g. 5-15 minutes worth of fuel).


For a normal flight this means you'll land with around 45-60 minutes of fuel on board, sometimes less.  You'll fill up for the next leg at this airport while you deplane and enplane passengers.

Fuel at the hub airports is sometimes a bit cheaper due to the fuel contracts the airline can secure when they order massive amounts of fuel every day.  Fuel at outstations that don't see a huge volume of fuel may be quite a bit more expensive.  In this case, and if your weights allow, the airline may elect to load extra fuel that isn't needed.  The purpose of this is so you do not need to buy as much fuel at higher prices at the outstation.  

I only saw this happen operationally when we were going to be very light and the outstation fuel was very expensive.  Usually our loads were high enough that we were struggling to get all 50 passengers, bags, and especially jumpseaters on board with the required fuel load.  In that case it would not be economical to take more fuel because we'd have to take fewer passengers.