Rewire is the connectivity between reason and protools or cubase or whatever, so that you can essentially use reason as a sound module and record/sequence/process the sounds from reason straight into your DAW. some people say that it causes loss of quality and just export their shit from reason as wav then import it into their DAW but i havent found this to be true...
look up youtube for how to set it up with your daw. it's not complicated, you just have to set the ins right...