Rewire has two main purposes.
One is to connect and control other slaves as instruments in the master.
The other one is to synchronize the transport controls in several apps.
In the first case you can for instance use Renoise as Master and reason as slave.
If you add a instrument in reason you will be able to control that instrument from renoise using Rewire Midi. The instrument will show up as a Midi device in renoise.
For the second use: if you start to play the song anywhere in cubase or in Renoise you will start both applications synchronized.
For the audiosignal: Slaves feed the master with audio channels. The master have control over the soundcard.
So if renoise is master you can select any channel from the slave in the ReWire Device in Renoise.
This way you can mix everything in the Rewire Master.
And last. Cubase can not be a ReWire Slave. It can be master only.
Please read more in the Rewire FAQ/How to
and
Available Rewire Slaves & Masters, & Compatibility with Renoise