Mass Breeds are events organized by members of the DC forums, where they all plan on breeding on the exact same day. Often, participants will only breed certain types of dragons (usually rares, such as metallics). This is usually done in hopes of producing many rares to gift to people.
There are mixed reactions to mass breeding, ranging from people who enjoy it to those who hate it.
The most (in)famous Mass Breed was that of Halloween 2008, where every single dragon in the wilderness and on participating users' scrolls were bred. This event, combined with many imitations that followed in quick succession, led to a massive cave block, with a backlog of thousands of eggs. This block continued with very brief openings for over a year and led to such an enormous tip of the black and (to a lesser extent) vine ratios that the shockwave is still felt today.
The idea of mass breeding originated because there were rarely eggs in the AP and not enough CB eggs to go around. This gave rise to the idea of spreading the love though breeding; initially, this was limited to a small number of people who made a habit of breeding specifically to abandon the eggs. Mass breeding is simply an organized, large-scale version of this idea. Because the egg shortage is now a thing of the past, this is no longer necessary and is generally considered a very poor idea. It still has a tendency to cause massive cave blocks, which only leads to frustration. While this is no longer an issue due to the AP auto-clean, this also means that the efforts are wasted when the eggs are automatically shoved into the wild.