Your iPad doesn’t store everything on some magical streaming cloud (or at least not yet); all your apps, music, books, and videos need to be put somewhere. To check how much more content your iPad can hold, go to Settings -> General -> About -> Storage Available. This will probably be displayed in GB. Note that the size of an app is displayed at the bottom of it’s page in the App Store (usually in MB; approx. 1000 MB = 1 GB), and most songs are about 1 MB per minute.
Have fun with your iPad!