The working of non inverting amplifier is similar to that of inverting amplifier except that the output has no phase shift.
The resistors Ri and Rf form a voltage divider network.
A negative feedback is provided by applying a little of output voltage to the inverting input terminal through the potential divider network Ri and Rf.
The voltage gain of the amplifier is determined by the ratios of Rf and Ri since Gain, A=1+Rf/Ri
So the amplitude of the output voltage signal can be varied by varying either of the resistors Rf or Ri.