What does the trouble code System too lean bank1 p0171 mean for Toyota sienna 2003?

It means that the rear bank (bank number 1 since cylinder 1 is the first cylinder on the passenger side in the back) is not getting enough fuel. A lot of the time, this trouble code is just an O2 sensor going bad, but here's a more explanative description: Trouble Code: P0171

Fuel System Too Lean (Bank 1) Possible Causes: * A/FS or HO2S is contaminated, deteriorated or it has failed * Air leaks after the MAF sensor, or in the EGR or PCV system * Base engine "mechanical" fault affecting one or more cylinders * Exhaust leaks located in front of the A/FS or HO2S location * Fuel system supplying too little fuel during cruise or idle (faulty fuel pump or fuel filter) * Fuel injector (one or more) dirty or pressure regulator has failed * Vehicle driven low on fuel or until it ran out of fuel