The early morning alms giving ceremony is a Luang Prabang tradition. Each day a procession of dozens of saffron robed monks snake through the center of town collecting donations of food for them to eat later in the day.
In other parts, monks go out in groups of one to three to collect alms. In Luang Prabang they do it on mass. I'm not sure why they all concentrate on one area instead of fanning out into residential areas but it certainly makes for a highly photogenic event.
There are a handful of simple rules if you will be attending. Be quiet, don't touch the monks, don't call out to them or otherwise try to get them to pose for your photographs, don't get in the way. Common sense stuff really. There is however one rule that seems to need repeating.
Do NOT use your flash! Seriously, sticking a flash in the faces of people making their way quietly through the early morning half light is incredibly rude. Not just to the monks but to the people participating in the ceremony. If you can't get a decent photograph using what natural light is available then accept it and just experience what's going on. Don't be surprised if no one says anything at the time. They're Buddhists, who are not typically given to telling strangers off. Now that I think about it, maybe they will even thank you for the opportunity to practice loving kindness in the face of adversity.