I'm working on a Flash project and someone recommended Red5 as a media streaming server.
Do I need to know java to use build applications that use Red5 as their server?
It depends on the project.
Red5 will be used for the server-side part and it comes with a number of features out of the box.
If for example it's just a simple video on demand with RTMP streaming, you can do it with Red5 and the oflaDemo plugin which has already and is very easy without touching java, but if you need something more complex for server-side (for example a playlist, use of cookies etc), then you'll need to know java and even then it will be pretty difficult as there isn't very good documentation for Red5 out there.
But you can control many things from the client side with flash.