ring_ready is a dialplan application that sends the protocol specific message to indicate ringing.  In the case of SIP a 180 Ringing.

ringback is a channel variable you can set to artificially generate a tone or play an audio file to an originating channel while it waits to be connected to another call.

pre_answer is a dialplan application that will send the protocol specific message to indicate early media.  In the case of SIP a 183 Progress

So, if you call into freeswitch with sip and the first entry in your dialplan is ring_ready followed by a bridge to some other destination when it hits ring_ready it will send "180 Ringing" back to your phone so it can generate the ringing sound.

If instead you use the set application to set ringback to a tone spec or audio file followed by a call to pre_answer to establish an early media connection followed by a call to bridge to another dest, then the core will generate this audio locally and send it back to your phone. Again, this occurs during early media meaning the call has not been answered but the SIP has negotiated a media path in advance for this type of pre-answer audio indication.  SIP has no promise that early media must be supported so some switches and devices opt to not support it meaning you may not be able to hear any audio until the call is officially answered which would keep you from hearing the artificial ringback at all.