Both devices need to be on the same WiFi network, and both have to be running the Command C app ($4 on the iOS device, free on the Mac.)
In order to copy and paste, you just copy whatever it is you want to share and then go to Command C and click the device you want to share it with. It's very simple and, so far, it seems to work well.
There's a similar product for Android & PC called Alt-C, but I haven't tried it. If you do, or if you have used other tools that do the same thing, I'd be interested in your experience.
There are other ways to accomplish similar things, but many of them are more tedious to use because of all the other things they do...