Question about adding an Amp / bi-amping speakers

[Hoser59]

Originally Posted by arrt vandelay

only problem is, how is the amp going to get the signal from your str-dg720? that model does not have any pre-outs for an external amplifier. the only way it is going to send a signal to your fronts is through its binding posts...

if you're really serious about adding an external amp, you need to update the receiver to a model that has pre-outs...

This is the part I was most confused about. I had read some about pre-outs but I wasn't entirely sure what thier function was. My receiver does have one set out audio outputs (2 line RCA), what is the purpose of these? I had assumed (incorrectly I suppose) that I could just plug these into the amp to pass the audio through