What res mean on a Yamaha wave blaster on the knob to turn on and off the fuel.?

Res means reserved fuel. The knob has three directions.

1.on.

2.off.

3.reserve.

For example consider your tank has a capacity of 10 liters. 7 liters of it is usable and the other 3 are reserve fuel. When u turn the knob to "on" you can use the fuel above reserve fuel level i.e all the 7 liters. When ur tank reaches the 3 liter level, you will not get any fuel supply if you turn the knob to "on". You have to turn it to "res" to be able to use the reserve fuel.