Political debates are generally designed to be carefully crafted events where candidates on opposing sides of the political spectrum can discuss various positions on a variety of issues that matter to voters preparing to go to the polls. Sometimes there are moments of levity where a candidate says something they intend to be funny - it is bad when the audience doesn't laugh at those moments. There are other moments where the candidate says something they mean to be serious - it is bad when the audience breaks out in hysterical laughter when you are mid sentence.
This video is a case of the latter - Corey Stewart tried to be serious and the audience didn't even try to stifle their collective mockery. What did Stewart say? He tried to pull the typical GOP / FOX News talking point: Donald Trump is "standing up to the Russians".
The full quote was a nice set up, Fox News style. Obama was weak on Russia, Trump is strong. Except he isn't. At all.
“He was noticeably silent when the Russians shot down a Malaysian airliner when President Obama was in office. He was noticeably silent when the Russians invaded the Crimea when President Obama was in office. We have a president who is standing up to the Russians, and now, Sen. Kaine…”
The audience took a pause and then erupted in mocking laughter. Never a good sign.