Usage rights tell the user what they can and cannot legally do with images that they wish to reuse.
A Brief Summary of Types of Usage Rights (from Google Support)
Find more information on this topic at:
NOTE: You may need to do your image search by License/Usage rights at these sites.
Hover over the title of the resource to get additional information. ALWAYS check images rights and restrictions prior to using an image.
Most images require at minimum an attribution, while some have other usage restrictions or requirements.
Use Creative Commons to look for images that have licenses that fit your use, or to see if you can locate the license for a particular image that you have.