Vote bombing means to vote biasedly or without proper justification.  Vote bombing occurs when a member casts what might be regarded as an unwarranted vote, usually indicated by that member giving an unsatisfactory reason for their decision to vote the way they did (e.g. "I agree with Pro." or "Con didn't refute Pro's case.")
The casting of vote bombs seems to have gone down since Juggle stopped making votes anonymous as well as since Juggle introduced the opportunity to require an RFD (reason for decision) when voting on a debate.
While the most popular way to counter-act the appearance of vote bombs has simply been to "counter" it by voting in favor of the opposing member, a new feature that allows users to report votes has rendered it obsolete. However, countering or reporting votes are seen by some to be also biased since there's no objective criterion which one can use to point out a vote bomb, therefore there's no clear line of demarcation between simply a poor RFD and an outright vote bomb.
Site rules on Vote Bombing
The website has a set of rules that are used to decide which votes ought to be removed. Votes based on personal biases are liable to being deleted. 
Alleged Instances of Vote Bombing
Debate.org user FrizzStammberger has accused the website's voters of having a pro-atheist slant. According to the user, the majority of the users on the website were atheists, and they were "hostile to Christianity". 
- ↑ http://www.debate.org/help/faq/
- ↑ http://www.debate.org/forums/Debate.org/topic/68208/
- ↑ http://www.debate.org/debates/There-are-more-atheists-than-theists-on-debate.org/1/
- ↑ http://www.debate.org/debates/Debate.org-is-crawling-with-atheists-hostile-to-christianity/1/
- ↑ http://www.debate.org/forums/Debate.org/topic/24052#1131095