To be basic, when something happens that leaves a character babbling like an idiot. This can be the result of something shocking being said, which would lead to having so many emotions and thoughts at once that it becomes hard to form a complete sentence, to times when something afflicting a person leaves them unable to communicate with others. This can be especially frequent for an embarrassed Tsundere, which can be a form of fan service. For some odd reason, when something like this happens, people don't always know why. This can be especially strange if their babbling is caused by embarrassment about a crush on someone.
Examples:Anime and Manga