Playing songs on my iTunes library on my iMac outside?
Just like Air Video, can I listen to the files on my iTunes library on my iMac even when I'm outside?
If it is possible, what app should I use for it?
iPad, iOS 8.1.2
You can do this as long as your WiFi network is strong enough. Use Home Sharing.
iTunes: Setting up Home Sharing on your computer - Apple Support