You could just find a version of the Creative Commons license that you like, and say your assets use that license. I wrote my own because license because I'm pickier
The license is more or less the terms of which you let someone use your assets. Most people who make assets for people will use Creative Commons with Commercial usage with attribution, one of the MIT licenses, or folks will try to write their own. It's important to include because it gives people permission to use it in their projects.
You need to pick out a license, or write your terms of use. The website linked under the drop down for the license requirement on the page has a sample link, and that site has a dozen you can choose from and just link directly in that box.
