iTunes Match is a subscription-based service made available by Apple. For $24.99 a year, iCloud will take all the music on your computer (even music imported into iTunes) and sync it to a private cloud. This allows your iPad to stream and download them all the time, any time. Once you have purchased and set up iTunes Match, you have one last step: turn it on on your iPad. To do so, go to Settings -> Music -> iTunes Match and flick the switch.
Have fun with your iPad!