A photo or other creative work that is considered in the public domain is one that has no copyright. That makes it free of copyright violation. Sometimes the copyright owner places the photo or artwork into the public domain; other times the copyright expires.
You may find many free photos online that are in the public domain. That means the field of use is wide open, whereas other free photos are still only available for a certain range of use. You should always know before you download a photo if it is in the public domain or not.
Also, keep in mind that images in the public domain may be used by lots of people. So that great shot you found for your blog or rotary newsletter may show up elsewhere and therefore not seem so special anymore. This is the chance you take when using a creation that has slipped into the public domain.