Op-amps designed to operate from a single power supply employ a dc insertion technique. My textbook says that it is to obtain a maximum undistorted symmetrical sine wave. What does this mean?

 

I never heard of "DC insertion Technique".
Any opamp works from a single supply voltage if its input is biased at half the supply voltage and the input and output are capacitively coupled.
Since the input is at half the supply voltage then the output of the opamp will also be at half the supply voltage can swing up and down equally.

 

It means that you have to bias the op amp as to output about half the supply voltage when no input signal presents. This is to achieve the maximum output voltage and a symmetrical waveform.