The CC stands for Creative Commons, the non-profit organization whose copyright licenses and guidelines (aka Creative Commons licenses) have been widely adopted—and most importantly, respected—by creators and the Internet community for the legal use and distribution of their work. Basically, the licenses detail what specific rights to the work a creator is willing to waive, and any restrictions they may require, for the work to be used for free, and even built upon.
That was a mouthful. For our purposes here, we’re looking for images we can use for free. CC Search lets you enter keyword(s), and it will present you with a bunch of images from all over the Net, each with a set of icons that communicate the aforementioned licenses for using the images free.
The good news is you can check off and limit the searches to specific types of licenses from a list. It’s the first two you’ll want for totally free and clear images, which are “public domain” licenses. These translate to you can do whatever you dang well please with them. The other licenses have varying requirements and restrictions, such as providing credit, not for commercial use, yada yada. I have zero issues with providing credit; it’s the non-commercial stuff I keep a lookout for and weed out.