While much of the Zedge community-contributed content is free, you'll also find lots of incredible and original Premium content that can be unlocked/purchased.
While some Premium content items can be unlocked by watching video ads. others may only be acquired by spending Zedge credits which are made available to purchase in bundles via in-app-purchase.
Access the Zedge credits menu by clicking on a Premium item that displays a Z amount or click the Z credit wallet amount at the upper right corner of the apps.
Select your bundle and complete the transaction via Google Play or Apple iTunes. Upon a successful charge, you should see a short animation showing the Z credit total increased. After successful funding, you'll be able to unlock items priced with Zedge credits.